I'd extend it. There are a lot of r56 minis out there with full mini history, and one without it obviously won't sell as well as one with - think about it you've got the choice of a car that has been maintained by a dealer, or a back street garage (in the eyes of someone who knows little about minis, other than they can have a different colour roof). So this person would probably buy the one with mini history because they know its been serviced right, and 'proper' parts have been used. Your car would therefore be harder to sell, so when it came to trading in, it would be worth less. This would be about £500 - something like that. As extending the TLC would be less than that, I'd do it, but use an independent for things such as changing brakes. Then you have full mini service history but aren't paying out unnecessarily

I'd do whats needed for a stamp in the book at a dealer and the rest elsewhere.
Oh and a warranty to factory standard (everything really) is £400 with a £100 excess, although there are cheaper versions which cover less etc.