Hi Lubos, the ones i'm aware of are below :-)
1) In the grounds of Borough Church "St George the Martyr" next to the railings (behind the phone box), this tree used to be huge but they keep hacking it back, which only seems to encourage it. Leave looks a bit like Dalmatie? there are bits to take cuttings from still.
Borough High St, London SE1 1JA
2) Byng place, outside the big church next to the phone boxes. No idea of variety.
Byng Pl Bloomsbury, London WC1E
3) Euston Station fig trees, there are a couple large standards, on the right side railings to the road entrance, I took cuttings of these, might be Osbourns prolific?
4) Hyde park marked on map, past public toilet opposite bench on "flower walk"
5) Queen Square, London WC1N, at the north end there is a huge old fig tree
6) smithfield, huge old old tree in the south side of the round garden at the center of smithfield car park (W Smithfield, London EC1A 9DY)
7) Outside the national gallery (north of trafalgar sq.) there is loads of small trees, there are two varieties as i've been there in summer and they have very different shaped leaves
8) At St James park, at the north end of the bridge crossing the lake, big old tree
9) Chelsea Bridge tree, apparently a unique seedling which is sold commercially. I went there during a lunchbreak on a cutting mission but there was a small locked gate, prob ok to jump over and take some cuttings if you're feeling athletic ;-) Its on the embankment area accessable from the roadside on the A3212, leads down a little path to the tree, I dont know the precise location)
10) A communal garden has many fig trees growing round the west and south edge, nelson square SE1
You can check them out on google maps to plan which ones look best, Happy cutting! :-)
Steve