Aildreda de Tamwurthe's persona is mid-13th century English. That presented a slight challenge, as all of the legal documents of mid-13th century England that I've found so far are in Latin. I still haven't mastered Latin well enough to write in it. I can translate out of it, laboriously, with a dictionary in hand, but I can't compose in it.
I started with a Latin legal text from the 1260s, used multiple translations to confirm my own sense of what it said, and then put the modern English into pre-1300 Middle English. The end result still reads a little more modern than I'd like, but I didn't want to hold up the scroll-making process while I fiddled to a standard of perfection that I probably couldn't meet.
The text contains deliberate internal spelling inconsistencies, mirroring the variability in spelling found in period documents. For example, I used every spelling of "and" found in Middle English prior to 1300, just as a present for Dreda.
The ȝ character is a yogh. It's pronounced like a modern 'y'. ð and þ are both pronounced like 'th'.
All of the footnoted definitions of terms are based on the definitions in the Middle English Dictionary.
Ioannes, by rihte of armes Kinge of Estlonde, & Ro Honig, by þe samen rihte þe Quene, ȝif gretunge to alle whamm þe present leattre schal rahte. For þe onour & þe auantage off oure selues ond off oure reaume, We, by þe consil of oure magnates, scilicet þe worþssipuol Ordre off þe Pellican, herbi & heonne-forthþeward astoll, staþel & bifeste Aildreda de Tamwurthe as maistresse of þe ordre afoure-named. Ant we do herbi conferme & ifaste þe leattres patents biforen-ȝifenn onto þe fornseid Aildreda, wið alle þe rihts, freodoms, freoscipes, remissiuns & largesce as schal belongeþ to a Peer off þis reaume. Ant we ferþer comaunde þat þe sygnacle & bagge of þe Pellican be beren her-æfter uppe on þe fornseid Aildredas persone euere-more, þe hwich commandement schal be obeied on peine of such deseritisun & remevement off benefices as we demað soðfeste & fair. Ant þis ure leattre preouyng þe samen we sened wiþ vre seel uppe-on þis twenty- seueþe dai of Mæi, in þe fifti- secunde ȝer.
 scilicet = that is, namely
 astoll = to set up, establish, institute
 staþel = to establish
 bifeste = to endow with, give
 ifaste = to confirm, as an agreement
 biforen-ȝifenn = previously given
 freoscipes = rights or privileges; also freedom from servitude or obligation
 remissiuns = immunities, freedoms
 largesce = freedom, liberality
 sygnacle = a seal, sign of genuineness, hallmark
 bagge = yep, that's how you spell badge in the 13th century
 deseritisun = deprivation or loss of possessions or privileges
 remevement = bet you've never been threatened by your own scroll before
 soðfeste = primary meaning is "real, actual; genuine, authentic;" it is used in here in its secondary meaning of proper, just and righteous