With regards to your proposal to launder the money, this won't work. To my fairly certain knowledge in order for the proceeds of gambling to tax exempt, the counter party (in this case Just-Dice) would need to have a gambling license (or equivilent for the jurisdiction in which they are registered). In effect, if you laundered your Bitcoins and received 39,500 GBP back, it would still be liable for tax and you'd just have given Just-Dice 500 GBP.
Thank you for your information:
So its only treated as gambling if the counter party has a recognised licence.
So if I can find a Bitcoin Gambling Site that has a license in some obscure jurisdiction like Belize or Costa Rica etc. or perhaps an existing gambling site may start accepting Bitcoin over the next year or two, I may be able to avoid the CGT then I can sell my coins at Bitstamp/Gox(not) to hopefully avoid the VAT.