You could end up in major hot water buying a property off plan. Basically at the start you're promising to buy the property when it's complete, you normally put down a deposit at this stage. You will also have to show a mortgage offer for the rest. However, it's far too long between this and the actual completion date for this mortgage offer to still be valid, so you need to get a new one at that point. If at that point, you can't get a mortgage, you're royally screwed as you're bound in a contract to pay £X on X date and if you can't pay, you will definitely lose at least your deposit. In addition, when the developer puts the property back on the market, if they can't get the same money from someone else, they can sue you for the difference.
This is a particularly big risk if you'll both be studying at completion date as I don't understand where you'd get a mortgage from at that time?
So in short, yes it would be idiotic to buy this house in your situation, especially if you don't even want to move into it then. You pay extra for a brand new house and, if you then rent it out to someone, it's not even going to be brand new when you move in. People never take as much care with rentals as with their own home. Why not just look for a house nearer the time you actually want it and save all the risk?