Having worked at the CAA PLD for 4 years prior to joining my airline, I'm pretty confident even the CAA use to at least recommend against this. I don't think they had the man power to really justify the time spent on this - even if it were at no cost to them.
If you're looking at doing an ATPL skill test observed which is what I guess you're talking about, on an rare aircraft with no sims approved in the UK then perhaps they would make an exception. If it's something like an observed test on a 737/320, I think they'll tell you to forget it, and get your butt over to the UK. Again call to check +44 1293 573700.
If you do get them out, don't forget they will be flying 1st class, staying in 5* hotels etc etc- so unless a company is paying for you, and money is of no real concern, it is far cheaper to go to the UK - and the CAA I'm pretty sure would (still) advice the same thing.
VJW