Most modern mains transformers have 2 windings on the primary, both rated for 115V. Connected in parallel they are fine for North America, connected in series they are suitable for the European (and elsewhere) 220-240Vac mains. Many amps have mains voltage selector switch which simply changes the series/parallel configuration. Without the selector switch, if the mains transformer has 2 primaries, it can still be done with a fairly simple wiring mod.
I'd be careful though, do your homework, pick the amp that you want and try to download the manuals or even better, the schematics to see if it is suitable.
Of course, if you really can't modify the amp, you can buy an external 240V to 115V transformer. This is not recommended as they're so heavy to lug around.
On the other part of your question, if you buy anything of value in the States (I think it's >$50) you're supposed to declare it at UK customs and pay VAT at 17.5%. However, you are exempt from local sales tax in the US when you buy. I know you can claim your sales tax back at the airport on your way out but I believe there's some way that the store can not charge it you in the first place. Perhaps one of our American cousins in the forum can explain how 'out-of-state' sales tax exemption works.
You may choose not to declare the amp at customs and 9 times out of 10, you'll just walk straight through the 'Nothing To Declare Lane'. If they do pull you, your best bet is to try to argue that you bought it in the UK and took it with you. However, those guys are not stupid, have seen it all before and will recognize a brand new amp. If they're not convinced they will impound the goods until you produce a valid UK receipt. If you can't do that, they will charge you 4 times the VAT (17.5% X 4 = 70%).
I got pulled at customs with a friend at customs returning from Thailand where he'd bought a camera at a fraction of the UK price. He told the customs guys that he'd bought it at Dixons in Manchester and initially customs accepted his story. But when they searched his suitcase, they found the receipt, guarantee etc - game over. If you're going to attempt this stunt, post the documentation to yourself from the States.
Despite these technical and legal challenges, the exchange rate is so good at the moment that it still might be worth it.
One last thought, watch out for excess baggage charges on the airline, many are tightening their limits at the moment.
BTW, I envious as hell, it's a couple of years since I've been in California.
Andy.
