Hey Jonny,
sounds like you're going the right route really ... school, uni, career - just avoid too much debt at uni! Student atco pay isn't what it was. The rewards are there though!
I wouldn't bother with the fiso thing, unless you can get someone to pay for it - summer job at a small aerodrome somewhere - I don't think its worth paying for yourself. (This is where I get abuse from someone who probably knows better than I do regarding FISO jobs!)
It will show you are keen, but so will doing some research on the internet on ATC - check the above sticky. FISO will give you some experience and you'll enjoy it too - hopefully! Could be another route into ATC too, its not just NATS that provides ATC inn the UK. Depending on where you live, you could try to get into assisting first, then go for an atc course (thats the way I did it - assistant/fiso, then atc). Thats just how it went for me. Good luck matey .. and you've plenty of years to go ... enjoy uni!!!
D.U.