Stu, as someone who was in the Forces and is now in the City I can vouch for the sense in joining up and leaving the grey and dismal office life behind. Money is not everything and that is all that the City offers. Why do you think there are so few middle aged to older people in the City? Folks make as much as they can for as long as they can stand it and then leave. It's a fact.
You've got one chance to join the Forces. Do it now while you can and you'll probably be glad you did. If you don't you'll always wonder. You can always come back to the City but you can't join the Forces if you miss the boat now.
If you join the Forces you'll meet great people, get to experience real camraderie and have a pride in yourself that you will definitely not have if you remain where you are. You'll get more stories out of your basic training than a life time in the City could afford.
All good advice on the previous postings however I would urge a note of caution on joining the AAC as a soldier in the hope that you can get selected for aircrew after four years. A lot get this carrot dangled in front of them but never get to realise their dream four years down the line. Personally with hindsight I'd take kbf1's advice and join under another cap badge and then do a secondment.