411A
Perhaps the 'best' option for the RAF to use their TriStars to Kaubl (or Bagram AFB) and then thereafter...civvy air transprt out of theatre, directly to the UK.
Please read the earlier posts. It is UK (not RAF) policy that all flights in Afghanistan (intra and inter) that carry UK military passengers are undertaken in aircraft equipped with DAS. It doesn't matter where these land (KAF, Bastion, Kabul, Bagram, MAS....). Therefore, if a civilian company was to do this job it would need DAS equipped aircraft and DAS equipped crews.
Now it would be different if we took your suggestion and used civilian air to transport personnel to an airport in a nearby country and then move them into Afg in a DAS equipped aircraft. Perhaps then the MOD (not the RAF) could charter civilian airlines to do the leg between the UK and a regional country. You may wish to contact the MOD (not the RAF) and suggest such a thing.