I've only had good service from Intersango so far, transfers and withdrawals have all taken up to a few hours to arrive / be credited.
I can also fully agree that British Banks are utterly useless...
One thing I think could be improved with Intersango though is in the Buy-Sell Interface, whereby currently you can only define amount to buy and price per unit; Mt. Gox on the other hand lets you put in any two of price per unit, number of units, or total price, and calculates the last one. E.g. when I want to buy £20 of bitcoin I would rather type in a price per bitcoin, and select £20 total price -- letting the page calculate the number of bitcoins I can buy, rather than select a price per bitcoin, then go calculate the number of bitcoins I can buy externally, before entering this number.