[excession]

I don't think playing online poker under age in the UK is illegal per se, just against TOS - the purpose of the laws against under age gambling is to protect the under-age person so it's most unlikely that the minor is himself committing a criminal offence.
Those under-age folks who occasionally 'win' the National Lottery don't get paid out, but they aren't arrested either...

Providing a fake ID to an offshore poker company isn't a great idea but off the top of my head the only applicable offence I can think of is obtaining monies by deception and as they are 'your' monies anyway I'm not so sure that anyone is going to care very much..the company is only asking for proof of ID to comply with money luandering regs not because they are hyper-vigilant about under age players