Your best bet is to use
www.transferwise.com to convert and move GBP to MTGOX SEPA EUR account, and trade in EUR.
I've seen where Mt. Gox only accepts bank transfers if it comes from your own account (and not a commercial account like Transferwise). Are you able to perform this SEPA transfer method from Transferwise to Mt. Gox and they take it?
There is no problem with doing this, as Transferwise are licensed by the FSA and HMRC, so they are seen as part of the financial trail. I have used Transferwise 3 times to move funds to MTGOX: GBP to EUR SEPA. Both Transferwise & MTGOX are totally aware and happy with each other. It's the best way.