BMW will be very expensive, only use them if you have no choice.
I have not needed new tyres yet, but have done a bit of research, just incase. The best price I've seen so far is
http://www.etyres.co.uk/, and they come out to you. Shame It's not a 24hr service. I haven't used them and so cannot vouch for their service.
I've yet to check out my local tyre dealer, but with only 150 miles range, I probably will not be able to be too fussy should I have a problem.
I did consider the Tyre/Key insurance which my dealer tried to sell me, but I felt that the limit on claims was too low. If I didn't hit that number of claims, I'd do better buying them myself, If I needed more, I'd end up buying them myself anyway.
With the state of the roads in London, I'm expecting trouble. You need a large 4x4 to navigate some of the roads around here.