Yo man, i live in the East End of London.

Theres plenty to do around both cities. Although Londons bigger and better obviously.

Theres plenty of headshops around London, if you can try Soho, theres a few down there. If not you could out into the boroughs abit away from central London and find a headshop. Most things are expensive, because our government suck, but our weed costs a little less than yours, so that's cheaper. If you went round canning town, all round east london and camden market you could probably find your pick of like 10 people who would sell you weed and not rip you off.

I wouldnt advice using public transport though. Especially the underground. The underground sucks ass. If your not in a carriage with 100 other people you've got nut cases blowing themselves up. The buses arnt too bad, because theres so damn many of them, but your likely to get stabbed by some stupid little kids [i saw some bloke gets stabbed in the back with a cross head screwdriver once, it's fucked]

But dont worry, that's worse case, im sure you'll enjoy it and enjoy what there is to do and buy.