I wouldn't worry about the hair (unless we are talking a full-blown Rasta job) but I would turn up in a collar and tie; it is a job interview after all!
I would consider punctuality, cleanliness (clean-shaven, hair, nails, shoes, shirt etc) and a good performance at interview to be more important than a short haircut (which can be changed in 10 minutes flat!).
Shirt and tie, suit if you've got it, I made the same mistake missed the word 'interview' and turned up in a shirt and trousers but no tie and felt totally under dressed I also got grilled on current affairs history and what I wanted to do in the army as well as other questions.
you mean that for all your gobbbing off and cockiness you're a *cue gagging* STUDENT?!?!? fcuking outrageous!
as to the question mate, shirt, tie and a #2 trim seemed to work for me. it even seems to have hid the fact that i'm largely a fat useless cnut
sh1t, shower, SHAVE, trousers, shirt, tie, be well deodourised and show up 10 minutes before time and you'll be fine. i wouldn't advise a full blown three-piece, becuse if it gets a bit warm and you get nervous, you will sweat and smell. a light coloured, loose shirt will do fine in this weather. smart and casual while still remaining formal.
Collar and tie etc always a good start.
When you get there remember it is a sort of interview and first impressions count for a lot - after all you are interrested in being an Officer in the British Army long term. You should find it interesting, as to some degree they are also selling you their product after all as an educated man post College or Uni the world is your oyster too. But also be honest an explain the situation and your life plan any AFCO staff member (Officer Advisor etc) worth his salt will aim to ensure the two requirements dove tail together i.e your life plan and the Armys requirements of you as a potential Officer.
Relax, go there ask questions and listen to what is said ensure you fuly understand the requirements both ways.
The rest as they say is up to you.
P.S If there is an OUTC at your chosen place of study then there is definately no harm in joining while you are there - good luck
UOTCs are not attached to any particular university, so any Student in Further Education can join there nearest UOTC. Having said that it may be that it would be easier to join a local TA unit if you study a long distance from the UOTC TAC.
Do a search of this site as there is certainly more than one thread comparing the two (I would but to be frank I'm lazy).
Good Luck with your interview and subsequent interviews.