Haven't done it personally as my employer required a move to the IAA years ago, but I know others who have. They've all used the Maltese CAA (TM-CAD), hopefully this helps..
- Submit paperwork to the UK CAA to verify your licence.
- Separately apply to the CAA for a copy of your ATPL exam results if you don't already have them.
- Complete an initial EASA Class 1 medical. This can be done in the UK but must be at an AeMC (not direct with an AME). I think the AeMC's in the UK have Austro Control approval. You'd need to apply for a medical record transfer to TM-CAD if they won't accept the initial paperwork.
- English Language Proficiency assessment. Can sometimes be done over Zoom with an LPE.
- ATPL Skill Test. Potential to do the ELP here if your TRE is also an LPE.
- Any ratings over and above the one for Skill Test need to be applied for separately.
My estimate for the above is £1500.