There are a few special rules about the rate of exchange to use and the treatment of exchange differences, but basically the sterling profit is almost certain to be charged to income tax as trading income rather than to capital gains tax.
You can download a free step-by-step guide at http://www.forexfromscratch.com - just type your name and email and they will email you the book.Basically, the book is a 5 step guide to starting in forex, covering everything from the basics to live trading accounts.It's a VERY interesting read... and it's free!