It is.
So if I buy the iPad3 and Applecare in the US, the warranty and aftercare will be fully honoured in the Uk?
Yes.
This Plan is offered and valid only in the fifty states of the United States of America, the District of Columbia and Canada. This Plan is not offered to persons who have not reached the age of majority. This Plan may not be available in all states or provinces, and is not available where prohibited by law.
Depending on the Apple product, the AppleCare Protection Plan is available at the Apple Online Store, Apple Retail Stores, and many Apple Authorized Resellers including many wireless service providers. Apple recommends that you purchase the AppleCare Protection Plan in your country of residence, because each AppleCare Protection Plan has terms and conditions specific to consumers in the country or region in which the plan is offered. Please see the AppleCare Protection Plan Terms and Conditions for complete details
This Plan is offered and valid only if you are a resident of the fifty states of the United States of America, the District of Columbia, Canada and other countries in which Apple or its affiliated companies offers the Plan that are set forth http://www.apple.com/legal/applecare/countrylist.html.

I'm almost certain you wouldn't be able to get a 3G iPad from the states anyway, as you would have to get it on either of the carriers here, Verizon and AT&T, both require you having a full contract with them (they don't offer a PAYG kind of service for iPhones of iPads), so they wouldn't allow you to get one anyway.
Without meaning to be an arse. Source? I'd hate to buy it all, have it break and have no comeback.
Me buying a MacBook Pro in the states while on holiday in 2006. It getting a logic board swap under warranty and a battery recall with no issues at the local Apple Store.
Apple may restrict service to the country where Apple or its Authorized Distributors originally sold the hardware product. Apple will provide warranty service either (i) at an Apple Retail or AASP location, where service is performed at the location, or the Apple Retail or AASP may send the product to an Apple repair service location for service, (ii) by sending you prepaid way bills (and if you no longer have the original packaging, Apple may send you packaging material) to enable you to ship the product to Apple’s repair service location for service, or (iii) by sending you user-installable new or refurbished replacement product
or parts to enable you to service or exchange your own product (“DIY Service”).