If I try to send bitcoins to an invalid public address (for example, changing one character from upper case to lower case) using the client, it tells me it's an invalid address. This is good.
What happens if I enter an invalid address at Britcoin on the withdraw screen? Will it act similarly, i.e. refuse the transaction and tell me it's invalid? I don't want to experiment because the withdraw amount has to be at least .5 BTC, I believe, and I'm not willing to risk it.
britcoin talks to a bitcoind to actually talk to the network. If britcoin attempts to construct a transaction to an address that is invalid (i.e. the checksum doesn't match) then it will get an error back from bitcoind and should pass this error onto the user.
You can check this by looking at the britcoin source
(since it's open source).
in the meantime, check out the FAQ