Lots of Uni's are now offering degree course by distance learning and there's no difference between studying distance learning or on campus as far as the award and degree's worth is concerned.
Just about all of the command courses in the forces now attract civvy quals - you just need to pay the registration fee (and pass the course obviously

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My experience of civvy companies is that once you get past 21/22 then they aren't looking at you as someone they can train/mould, they'll be looking at you as an experienced hire. A few years in the mob plus a management qual and/or a self study degree will get you the interview, and to be honest that's about all it will ever do.
Why only look to join for 4 years tho? If you going to join then at least join with the intention of doing 22, you may change you mind once you're in, but I'd be buggered if I'd go through Lympstone again if I only had plans to do 4 years
Only caveat I'd say is that if you are looking to go on a graduate recruitment program then think carefully as a lot of them do have age limitations.