Swings and roundabouts I'm afraid. There are a number of suppliers/stockists of both aftermarket and GM/Ford crate engines in the UK. As retailers they should offer a warranty (as will a US supplier - but what if engine fails - who pays for shipping back to the USA?)
There is a 'history' of people sourcing high performance drag race and street 'crate' engines from the 'states and being disappointed when finding-out that the hp/tq figures stated by the suppliers/manufacturers fall short of supposed 'live' dyno quoted figures.
The US suppliers tend to say its UK fuel, or barometer readings and compensation figures that account for the differences - strange that most dyno facilities in the UK use exacty the same equipment as the Americans.
Even GM and Ford's HP/TQ figures are arbitary and make little reference to what fueling and exhaust systems were used for their quoted figures.
However if all you need is a stock 'cooking' engine you shouldn't have an issues - price-up your UK options and then compare against 'stateside' choices.
Me? I had my built here in the UK by a known specialist and he dyno'd it to prove his earlier estimates.