muyuu: You can trade GBP with direct GBP bank funding in MtGox. Currently Intersango's GBP trade volume usually matches and exceeds MtGox's however the actual bitcoin prices are a lot better on MtGox, especially with discounts applied. How do you deal in dollars? I've been looking into it but haven't found a cost effective way of converting and transferring my currency.
That's the problem, it's not cost effective. I have an account in dollars and another one (abroad) in Euros, however my salary comes to my British account in GBP and that's what I prefer to use. Transfers and currency conversion usually cost a lot more than fees from either MtGox or Intersango and also trounce the spread difference. Beating costs associated to deposits and withdrawals is basically a matter of volume, for trading small amounts making a free UK deposit to my Intersango account can't be beaten. MtGox and its £5/£15 fees would make sense for £10k+ and I'm not trading that much. However, if you don't plan to move funds much you can still justify trading in several currencies and with some arbitrage you'd come at an advantage after a bunch of trades. Again, I'm not speculating that much.
I'm looking into forex trying to find a way to get funds converted and moved into exchanges more effectively, in a few months I will be trading about £5k which is not much, but it's enough to look more into this.