canary wharf cash machines do.
ive never had a £50 declined, i think it might be illegal to do so (so long as its for a reasonable amount ie not using a 50 to get a 80p bus ride or something).. but then again i guess stamps are also legal tender - not sure how that works now that 1st class no longer have values on them
TBH i use a lot of 50's in this kind of way purely to break them down due to the lack of acceptance in a lot of places. Always try a 50 first, then if they dont like it use a 20 which ive had in change elsewhere.