EIPCW
Assuming you hold a UK issued CPL you do not have to do the whole MEP course, it is training as required by the Head of training at the ATO depending on your experience and length of expiry. That could be 1 sim session or may be several hours. Renewing the MEP can be done in the same flight as the IR and only adds about 15 mins to the flight to do the required manoeuvres.
Once you pass the LPC you will need to have the CAA convert your licence and issue you with an EASA CPL with the valid MEP and IR added, for this there is a fee, if I recall about £80 per rating but check the scheme of charges.
You do not need to do a full mep course!