As noted by Sammy Chadwick, much-filmed London Transport RTW75 was used:
Leon Daniels recalls:"Simon Kaye did the driving for us on the moving Great Muppet Caper scenes. We made some RT seat frames with backs but no cushions for the puppeteers to work underneath......I ended up in that scene with Kermit and Miss Piggy on bikes (Battersea Park) - look carefully in the background for some poor person in a rowing
boat. Every time we went back to our marks the 'cherry picker' just moved Kermit and Miss Piggy in a couple of minutes. I had to row the boat at full speed back and turn around!!"
Not so easy to identify is this beast, registration NAR216L, a sort-of doubledecker with a trailer: