At the present time, UK gambling winnings are not subject to income nor capital gains taxes.
Until someone in authority (or a court case) establishes that poker is a game of skill - and therefore could be considered a skilled trade profession - rather than a game of luck, it is considered a form of gambling and therefore
exempt from tax.
The case mentioned on the HMRC page
BIM22017 - Trade: exceptions & alternatives: betting and gambling: the professional gambler combined with
BIM22016 - Trade: exceptions & alternatives: betting and gambling: what is a bet? would consider a "Professional Poker Player" as someone 'addicted to poker'.
Hmmm...
Man guilty in poker skills case, the jury decided poker is a game of skill and chance combined.
Poker under the Gambling Act 2005 also considers poker a game of both skill and chance (or equal chance). Some chance involved = a form of gambling.
The only UK taxes that would be payable for poker would be Gaming Duty or Remote Gaming Duty (neither of which apply to poker players unless you are playing poker as part of business - that is, you're a representative of the card room/pub/club/etc at the place that poker is being played).
If the online site is US-Based, you may be liable to [url=http://www.irs.gov/businesses/small/international/article/0,,id=106252,00.html[/url]US taxes[/url] on winnings unless you fill in [url=http://www.irs.gov/pub/irs-pdf/fw8ben.pdf[/url]Form W-8BEN (Rev. February 2006) [PDF][/url]
One other thing, that I'm not sure of from your original post, is if you are a UK resident or not. If not, then you may be liable to pay taxes on your winnings in your country of residence.
BTW, I'm not a lawyer.
John.