Good to see you wanting to get qual'd up mate.
In answer to the questions regarding equivalences of qualifications and what employers are looking for have a look at the
National Qualifications Framework.
You will see that A levels are at level 3 and GCSE A-C are at level 2. If you visit your AEC they will put you through level 2 literacy and numeracy for free anyway and it has been mandatory for some time now for all soldiers to be at level one before going on CLM part 1.
What you've got to ask yourself though bud is why do you want to do them......you're already in a steady, secure job which will keep you until you are at an age where your A levels wouldn't be worth a damn anyway.
If, however, you are thinking of getting out very soon then speak to the AEC and they will assist you in re-sitting.
If you've got a spare few years on your hands though (and studying on Op tours isn't as difficult as you might think as long as you are dedicated) then as Chocolate Frog says - OU degrees are ideal.
PM me if you want to chat further mate, either about this, SLC, ELC or anything else - I've been doing it for years mucker (in fact I've spent ALL of my Fcuking ELCs already!!!)
