Are Gambling Winning Taxable in the UK
The short answer is that no, in general gambling winnings are not taxable in the UK. Tax is deducted at source from the gambling companies so the good news is that you can keep every penny you win. Even professional gamblers are not liable to pay tax on their winnings as the case of Graham v Green  proved. However, should a professional gambler - ie a high profile poker player for example carry on other business activities such a paid public appearances then they may be liable to pay income tax on the money they receive for such services.
At the same time, you cannot therefore claim tax relief on any losses made through gambling. If you receive money from referring others to casino or gambling sites as an introducer or affiliate, then any earnings you make from this would be subject to income tax in the UK.
The above applies to the ordinary gambler, bookmakers for example are classed as carrying on a trade, and therefore are liable to pay tax on their profits.
Gambling Taxes in the USA
In the United States gambling winnings are taxable both by the IRS and at state level. All winnings from all forms of gambling are classed as taxable income and must be declared on the tax return. Losses are also deductable and must be itemized on the tax form, and can be offset against winnings up to the total amount of winnings declared. Professional gamblers are classed as self employed under Schedule C
Gambling Taxes in Europe
In most countries in Europe, gamblers do not pay tax on their winnings. Countries with zero taxation on winnings include Denmark, Finland, Austria, Belgium, Bulgaria, Czech Republic, Germany, Hungary, Italy, Luxembourg, Malta, Romania, Sweden and the UK. In Spain gamblers have to declare their winnings as income. European countries which do tax gambling winnings include France (in land based casinos only), Latvia (25%), Netherlands and Portugal (lottery winning only), Slovenia (50% lottery tax - ouch!), and Greece - 10% on lottery winnings.
Declaring Gambling Winnings to the Inland Revenue HMRC in the uk
There is no requirement for gambling winnings to be declared as income in the UK. However, it is always a good idea for any gambler to keep accurate records of any income from gambling in case of an investigation by the inland revenue at a later point. Any large sums which are deposited in UK banks can auto trigger a report from the bank to HMRC and keeping accurare records of dates and wins can make explaining your sudden wealth a lot easier!