Do you really need a 5DMKII/6D for pictures of people's mouths? It just seems overkill for a subject matter that will 1.) be brightly light (especially with a ring flash) and 2.) not moving. Most entry level DSLRs will work just fine for that at under 1000 pounds.
Definitely go with high quality glass, though.
Yes I will be using it for leisure too. But with regards to the mouth I've seen pictures taken with full frame high level DSLRs and lower end ones with the same glass and there are differences in quality. I'd be using these pictures not just as legal records but will likely use them in promotional books and case presentations too. The one thing I was told and I agree with you here is the glass is important. I think I've chosen a pretty good lens and then I want to get the best camera I can afford alongside it hence looking at the 6d now.