The swap is very common in the US so its cheap there, if your patient you can look for used parts in the US, but dont forget shipping and import duties. Engines here can be had for 1k ish but they will likely have zero history, that and installing a box with worn parts (which you may not know at the time) WILL cost you money later.
For example to buy new, approximate prices maybe plus shipping and import (of course there are many other manufacturers to choose from):
EKK2 mounts £550
DSS 2.9 axles £750
Fuel rail £130
FPR £160
ECU harness £150
Header £350 (basic)
ECU £500-1000
Engine/box £1200-3000
Lots of other parts needed, plus labour if somebody else is doing it for you, so just be wary if quoted a low price, but if it can be done do it.