Hi Mizar,
I would reccomend the Premier Inn at terminal 4. It's the most convenient option imo. Get yourself a business account and the standby rate is £51,-. If you're lucky and book early you can get the non-flexible rate for £29,- (that's not via the business website). It's a 20 min walk to the training centre or 5 mins by car/bus. Rooms on 2nd and 3rd floor have fridges and irons.