If you're talking about the odd £20 here and there, then don't. Even if revenue gets to hear about it someway or another, they're only likely to query it with you if they think it's a reasonable amount of tax due. Anytime that happens (if it does) just tell them it costs you more in travel/equipment/drinks etc for practices as well as gigs than you earn and they're highly unlikely to be interested. If all those £20's add up to a few thousand a year over several years, then be prepared to pay up *something* if they ever investigate.
You *really* don't want to go down the VAT route unless you're incurring lots of expenses which would allow you to claim back more VAT than you charge and once registered, you have to handle the admin.