As I said before, you can do a type rating LPC/IR with any approved JAA LPC examiner and provided you have a UK national licence with all the other requirements it will open your ATPL. If you need a JAA licence you then transfer from UK ATPL to JAA ATPL for the appropriate fee.
JAA on the other hand require all the above plus a tick in the ATPL-LST box on the SRG1187 form. Only an CAA approved ATPL-LST examiner can tick that box. The reason it is different is that he is issuing you with a licence to command not just authority to sit in the seat.
If you book that in advance then FSI will arrange it.
Not complicated at all, just requires a bit of researchl
I know this because I have just done it, and had the same problems.
