Where to start? Good question. Maybe for the sake of getting something down I'll put in a few of the overhearings and observations that have amused/ startled me:

There was this vision walking into the tube station ahead of me a few weeks ago: bit like a younger version of Gregor Fisher, but with a mohican haircut and pubic beard, and sundry piercings about the head. He wore bondage trousers (the sort with a back flap and a strap connecting the legs) and hefting boots with a good deal of metalware about them. The first woman I came to by the ticket office looked a bit shellshocked, and I nodded in his direction as he moved toward the escalator and commented "Quite a vision, eh?" She nodded, and then said impressively "But did you see the front?" "No, I was behind him" Her eyes widened and she said " 'E 'ad a fong!" I possibly looked a bit blank, so she added "Y'know, like a woman's fong ("Oh, a thong, I see") - and it was all spiked!!!!" "Ooer - bit of a challenge, then?" I saw the front later on, when I got down to the platform, and it wasn't so much a fong as a pair of trunks with spikes, including between the legs, apparently. Come to think of it, I had noticed that he had a particularly wide-legged rolling walk, so I suppose that was the cause...

The weather not having been too brilliant last week, a concerned dog owner was moved to ask someone he met in the street " 'Ere, mate wheredya get ya dog's coat?" (as in waterproof fabric, not fur). "Scotland" came the reply. "I 'adn't fought of going that far" "No, tell yer what, there's one place I know does 'em for awsses, an' they do 'em for dogs an' all" "Awsses?" "Yeah, you know, like them blankets they put over 'em - they do 'em smaller for dogs".

I love quasi-official notices - the sort of thing that somebody in management concocts and puts up in a hurry before someone can complain, or worse, sue, but which nobody puts through a sense check first:
"The main public toilets on the station are closed from 8th March for approximately eight weeks whilst major refurbishment is taking place. Alternative facilities are available in the roadway by Platform 1"
(Liverpool Street station)
"As a result of vandalism, please take extra care when using the station footbridge"
(Greenford Station). (Note: This is extreme vandalism - Greenford station doesn't have a footbridge...)