Yea, getting used to the traffic being on the 'wrong side' is a *****! When I went back to the states last year to visit family, the first thing I did was turn into the oncoming lane! Luckily nothing was coming.
Riding in London is great, the road layout is 'organic', as they say. Not too many straight streets. And it's easy to get turned around and lost at first, learn the post code system and it's pie. There's enough cyclists around that most cars are used to us and since the big red bus lanes double as cycle lanes you can get a big 10ft wide lane to yourself!
As for messenger work, Autumn is definitely the time to look for it, unfortunately not too many firms do part-timers, a few but not many. Creative, Pink Express and Go-Betweens are the only ones that come to mind.

Any idea what part you'll be staying?