To make it cleared - I am not transferring, but actually departing from London (T5).
Would that still mean that I need to get to T5, move by bus to T5A via T5C and than transfer to T3?
Or would I be able to clear the security directly at T3 (would they let me in?) and then get airside to T5 (by going to arrivals using escort and taking a bus)?

Anyways, spending 2 hours transferring is definitely not something I am looking for. I just strongly prefer the Centurion lounge to the Aspire lounge so was hoping it would be just an easy bus transfer.