Income from Personal Gambling Activities is exempt from Tax in the UK. In the old days, you would have paid your Tax on your bet, or on your winnings, but I think that since that Tax was abolished, the bookies have to pay a Tax instead.
If you are running a system, a syndicate, or running your gambling as a business then it is a taxable income.
I belong to a Taxation forum, and an accountant said that if IR made you pay Tax on your winnings, then it is logical that you would be able to claim back your losses. Because the majority of people lose when they gamble, it would give Inland Revenue a large headache to deal with all the claims.
The letter that I received from IR encouraged me to pay my Stamp, which is fair enough as I wanted to make sure my health care is paid for.