Mundane Politics
The kingdom of England is a state of turmoil. Decades of upheaval and war have caused many shifts in the kingdom's fortunes. Most recently, the King of France's invasion and withdrawal and King John's death have left the kingdom in the hands of a minor, King Henry III (b. 1207). As a result, many barons refuse to heed to the Regency in place, leaving England, Wales, and parts of Ireland without a strong leader and many headstrong local nobility.
The Covenant's Overlord
England (and by extension, Wales and Ireland) does not have a custom of allodial land. Every square inch ruled by the king of England answers to an overlord, who ultimately answers to the King of England. This includes the covenant of Nova Semitae. Due to their position on the 'Marches', and the historic conflict between England and Wales, the covenant has managed to avoid paying taxes for many years. The lack of strong leaders in the adjacent shire (England) or principality (Wales) has played into the covenant's advantage.
Despite this tumultuous situation, the question still hangs overhead as to whom the covenant owes its taxes to. At some point, this question will need to be answered before the answer comes to the covenant demanding taxes. There are several options available to the magi. All of these, of course, open the magi up to charges at Tribunal for 'interference with mundanes', but such is the life of a magus in England.
Given the size of the covenant (400 people and growing!) it is now roughly township-sized, if spread out more than other towns of its population-size. This population size makes it possible to argue for a town charter from one of the following powers:
Countess of Shrewsbury
The de Belleme family are the current inhabitants in Shrewsbury Castle, having driven out the Welsh in the recent past. However, the earldom had been intended for another noble. This was never finalized, as King John died before getting around to it. With King Henry III, the child-king, on the throne, there is little to be done as any Acts he may do require his majority. As such, the de Belleme family will hold the Earldom until Henry III decides to take up the issue in the future.
Possibilities: Leverage the de Belleme family's precarious position, either for or against them. Ally with the rival family who was originally promised the Earldom. Ally with either family, or with the King in order to gain greater independence via a town charter.
Prince of Powys Wenwynwyn, Llew
The current Prince of Powys Wenwynwyn is Gruffydd ap Gwenwynwyn. Strictly speaking, the covenant is on the west side of the Severn, and therefore within the boundaries of the principality. However with the Prince being forced to conceded to Llywelyn the Great after his father's defeat, he is hungry to regain power, his land, and revenge. He is seeking allies on both sides of the border that could help him. Of course, the magi could always just... move the river and be on the east of the Severn, too.
Llywelyn, on the other hand, holds the principality by the skin of his teeth. While he won a decisive victory, he has rebellious populations to deal with in all his conquered lands, other Princes who still oppose his ascension as the chief Prince of Wales, and the English king to whom he owes nominal fealty. A stable town within an unstable province may be just what he needs to help secure his borders.
Possibilities: Leverage Gruffydd ap Gwenwynwyn's position vis-a-vis Llewelyn the Great, for or against him. Ally with either Prince in order to gain greater independence via town charter.
King Henry III
Although King Henry III is still a minor, a Royal Council does hold his power for him, though they are quite weakened by their inability to force the independent-minded barons into their will as only the king can compel them through their vows of fealty. With the recent withdrawal of French forces, who had occupied half the country for several years, the King's power is precarious and he needs all the allies he can get.
During historic periods of instability, rich towns have often managed to win a Town Charter granted by the King. In exchange for a large payment, or regular payments directly to the king, a town may be granted a Town Charter which frees a town from any local nobles' demands for taxes. Towns may run themselves with their own government and answers directly to the king. At least one other covenant, Burnham, has done this though not without controversy among the other covenants. Charges of interference with mundanes is almost certain at Tribunal, but such arrangements are highly sought-after and extremely lucrative to those covenants that manage to navigate the political terrain.
Possibilities: Gain a town charter from the King while avoiding the ire of the local nobles.
The Church
Over the years, the Church has been granted land through the wills of many nobles. In many parts of Europe, the local bishop, or abbot, are not merely leaders in the Church but are also feudal overlords. This is also true in England. A local monastery with considerable land-holdings on both sides of the English-Welsh border is not too far away and it's conceivable that the land the covenant sits on has been willed to the Church. (Or such a will could be forged.) This could remove the covenant from the nobles' wars as the covenant's lands would be owed to the Church, giving them an ally in dealing with the local nobles. Due to the monastery's rule of keeping their brothers (or sisters) within their communities, interference is lower. Of course, they may wish to concern themselves with the souls of their tenants and send missionaries to preach.
Alternatively, a local chapterhouse of the Knights Templar could serve the same function as many nobles willed land to the Knights who subsequently achieved land-holding status much like monasteries.
Possibilities: Arrange a relationship with the Church whereby the covenant owes its fealty and taxes to a nearby monastery or Knights Templar chapterhouse. Alternatively, gain a town charter from either a lord bishop, lord abbot, or the Knights Templar.
The Covenant's Bishop
Much like how the covenant sits at the border of two temporal lords, it also sits at the conjunction of three bishoprics: two English, one Welsh. To the west is the Welsh bishopric of St. Asaph's; to the northeast is the Bishopric of Chester; to the southwest is the bishopric of Hereford. The bishoprics of St. Asaph's and Hereford fall under the archdiocese of Canterbury, while the bishopric of Chester falls under the archdiocese of York.
Exactly who the covenant will answer to will depend on which way they decide to go with their feudal lord and how they decide their politics with the local clergy. While it may be possible to choose, say, a Welsh lord and an English diocese, it would introduce additional complications in their mundane relationships.