Depending on how much you drive, 1500 miles can take a while to come around.
However, if you need a courtesy car then you'll have to give them a few weeks notice for them to have one available.
You can also 'go over' your inspection due mileage by about 1000 miles, so it may be that you don't need one for 3 months.
As for the MOT, check whether the dealership is the MOT test centre, or whether they get a local MOT tester in. If the former then they may use the MOT as an opportunity to 'create' work for themselves (i.e. warnings of 'unroadworthy' tyres, pads, discs when they actually mean they've still got half their life left), and if the latter you might as well take it to them direct and save a few quid.