Getting the ATPL Skill Test observed at an overseas TRTO is probably not a realistic option as you would have to pay all of the the costs incurred by the CAA examiner (who will travel on a fully flexible business class ticket and stay in at least a 4* hotel) as well as a daily rate for all of the time he or she spends on the task (including travelling time). You're talking well in excess of GBP10,000.
It's not so much a matter of getting the examiner to turn up to a UK TRTO as getting him on the day you want him. There are not that many Flight Ops Training Inspectors for each type and so you need to book early to avoid disappointment. I freelance for a few UK TRTOs and they don't seem to have too much of a problem arranging observations for their clients.