Had proper tyre levers and a hand press for breaking beads at my old job, even then it was fucking difficult, if it's a low-ish profile tyre then you have no chance. Plus even if you get the new tyres on you will need to get them balanced anyway. You get over to corsham much? Take them into corsham tyres, they charge £1 per inch to swap tyres, I should imagine they'll do it cheaper if the wheels aren't on the car.