I've just got back from a couple of days in London which involved a lot of travelling and photographing on various forms of transport, and surprisingly only had one problem. This was a Metroline driver who objected to me taking a few shots of Harrow Weald depot, claiming that it was against the law. I pointed out I was on public property (I was actually on the opposite side of the road to the depot) so there was nothing he could do to stop me. He then said something about needing permission to photograph him, at which point I told him that the law said that you don't need permission to photograph anyone in a public place or where privacy cannot reasonably be expected (so standing right on the edge of private property, clearly in view from the public highway, does not give a right to not be photographed), and that I wasn't photographing him anyway, bus the buses and the depot. At this point he walked off. The only photograph he was actually in was a long shot of the whole depot frontage, where he isn't even identifiable.
Other than that, no problems. I did ask at Victoria bus station as there were TfL staff around and was just told "keep of the roads", and even was allowed onto the SET platforms at St Pancras to get some photos of Javelins. A London Underground driver walked to the other side of the cab to tell me that Sarah Siddons was out on a special (annoyingly in the wrong place for me to see it) and an LUL cleaner waited whilst I photographed an A stock at Uxbridge. Didn't photograph much on NR stations though as most of them arn't that good for photos, and there is little of interest (for me that's mostly new stuff, old stuff or ones I've travelled on or am about to travel on).