For the US ATP checkride you do not need an instructor sign off.
If you meet the requirements and have passed the written test you can take the checkride with (forgot his name) FAA designated Pilot Examiner who resides in England.
If you fail then you need a sign off from a FAA certificated Instructor of which there are plenty in the UK.
Besides, best of my knowledge you don't need a visa for an ATP checkride, at least if the aircraft is below 12,500lbs you don't.