Hi English Patient,
Please don't confuse validity with currency.
At the start date for your first multi-pilot aeroplane (MPA) course you must have a current multi-engine rating ie. within the last year (or is it 13 months? Been a while and airline LPC/OPCs last a year).
Sounds like any airline course won't be your first MPA so may not apply to you. As with all these issues a written confirmation from the CAA FCL is the only certain way of knowing. Apply/ask with plenty of time to go as their turnaround time for paperwork is months not days due to workload.
Good luck, esp when dealing with the CAA!