Depends if your charter airline is flying G-reg a/c over 45t and/or to the US under FAR129.
FAR129 a/c must already have secondary locks (phase1 or 2). Phase 1 could be a simple shoot bolt or a bar.
UK G-reg a/c must (soon) have the same (DfT Ruling).
The cockpit door lock could be u/s so long as the secondary device was ok.
UK MEL's should reflect this and are probably different to MMEL
