I have a Tradestation Account and simply wired money over (It's called a SWIFT transfer in the UK).
There were no problems and all went fairly simply.
When I transfered the money, I was given an estimated conversion rate. A few hours later, I was told the actual rate that it was processed.
When transferring the money I was asked if I wanted to transfer in US$ or £Sterling eg. do I want to transfer $30,000 or £20,000, I choose the dollar amount. By choosing this, I didn't know exactly how much it would cost me for a few hours, make sure that there is a bit of room in your account for fluctuations if you do it this way.
I transfered the money at about 9.30 am and it was in my Tradestation account by later on that night. The bank told me it would be much longer so I was happy 🙂