I know it was a while since the initial question was asked, but I though that I would follow up with another post to help anyone else looking through the forum for information on this issue...
I recently opened an account with Trade Station (the broker and software company). They are a US broker, so want funding in US$. I simply went to my bank, well building society actually, and transfered money from my account directly into Trade Station's bank.
In the US, this is called 'Wire Transfer', my UK building society call it 'SWIFT transfer'.
Looking at the rate they gave me and comparing it to the rate at that time of the day on a forex chart, I believe they gave me the current 'Interbank' rate.
There was a charge for the SWIFT transfer - £20. But, considering that this was an initial funding for a pattern day trading account, the £20 is not a sizable fee.
The transfer process was simple and fast. I initiated the transfer at about 9.30am UK time and I got an e-mail from Tradestation at about 11.00pm that night saying that they had received my transfer.
As I feel that the value of the dollar could well drop even further against the pound, I may hedge my account by shorting the £/$ in my spread bet account (I don't have a dedicated forex account).
When receiving funds from my trading account into my UK account, I can have them transfered in a similar way. Yes, I do need to think about the current value of the dollar against the pound, but as I said, I may hedge against fluctuations with my spread bet account.