I assume this will be used for a daily (or at least frequent) commute. How extensive is the journey and over what terrain?
What conditions do you expect to cycle in? Will you ride in all conditions or just fair weather?
What stuff will you be carrying with you? Laptops, books & paperwork, lunch etc.
IMHO £250 is nowhere near enough to spend on a bike that will be used daily in all conditions, although you should be able to buy something very good second-hand for that money. A lot of people splash out big bucks on a nice bike for commuting only to flog it when they discover that they don't really fancy it
If you buy a mountain bike, you'll eventually have to fork out for road-specific tyres and mudguards at the very least. I'd only use a mountain bike for a commute if I had to ride across fields. If your journey includes some hardpack trails, then a hybrid would be more appropriate but if the journey is on tarmac roads then a road bike is much better. You do not want suspension for road use.
I use a Cannondale mountain bike with no suspension, but it's been modified for road use: very narrow wheels and slick high-pressure tyres, mudguards and longer gearing.
Tell us more about what you will be doing with it.