[inquisitor]

I've used tuyo and except of the bad English of the Hispanic customer care agents and the inability to disable the mailbox I was happy with them.
As you might have read on the previous pages, Tuyo uses T-Mobile's GSM1900 network, so coverage is generally inferior to those carriers owning GSM850-spectrum. But that's only relevant if you are travelling to distant rural areas.
On eBay.com you'll find a lot of American sellers that sell tuyo SIMs including shipping to Europe for $10 or less.
Btw you can activate the SIM by calling customer care already from the UK, so you'll know your American phone number even before you travel and so you can set up an announcement on your British mailbox.