I just found two of the neatest sources when it comes to English patents and charters: Rotuli litterarum patentium in Turri Londinensi asservati and Rotuli Chartarum in Turri Londinensi asservati (if you can't get free PDFs from these links, try replacing ".de" with your country's domain).
The introduction of the first contains, starting on p. iv, "forms of letters patent in the reign of King John", giving examples of the following:
- safe conduct
- de rato
- de passu
- de intendendo
- of homage
and "miscellaneous", perfect for giving models of salutations and verbiage related to both praise and blame. (The summonses would be wonderful for letters of writ for peerages).
The second also has a tremendous introduction that works you through a formulary of the English charter, introducing each section that is included, what it's purpose was, and providing textual examples. Most useful for SCA text writing purposes are sections 7-9, starting on p. xxx. Section 7 is in "data per manum cancellari" and "data per manum nostrum", that is, whether the charters were issued by the hand of the king or his chancellor. As with patents of arms later in period, many recognitions didn't actually come from the king though they were granted with his approval and permission or at his request. This is something that doesn't often get reflected in SCA texts, where everything is written as coming directly from the granting rulers. I was lucky enough recently that TRM Sven and Siobhan were happy to deviate from this practice and allowed me to write a grant of arms text which came from the Principal Herald rather than the K&Q. Nevertheless, before writing a text like that, I would caution approving the idea with the granting rulers first.
Section 8 covers the datal clause, which discusses when anno domini dates were used, and when regnal dates were used, how months were referred to, whether the place was mentioned, etc., from the Anglo-Saxon charters down to the present time.
Section 9 is about the sealing clause, discussing the different ways that "sigillum" was used (it didn't always indicate a wax seal), how the Anglo-Saxon kings ratified their charters, what types of non-signature marks were used, whether the ratification came before or after the dates, etc.
The actual texts of both books is nothing more than charter after charter, patent after patent -- all in heavily abbreviated Latin, so it would take some work to uncompressed the information, but, still, wow. I'm sure I'm not the only one who will have fun with these!