Exascot, that's substitution pay. You wear your own rank but get paid as if you were your boss' rank.
MOSTAFA - My experience is quite different. Substantive Flt Lt, 2 years as acting Sqn Ldr on various operations, return to being Flt Lt back in UK, get promoted and started on Sqn Ldr pay band 3. Time spend as a/Sqn Ldr counted for the incremental pay.