Hope someone can help me

First off I received a email about a week ago to say that thompson are changing our flight times to a earlier flight (which was fanastic as we get a full day in prague on the friday!).
But now they have sent a letter to say they are cancelling the flight, phoned up today to see why and to ask why we couldn't be moved to another airport (as the letter states they have gone down every avenue, yeah whatever!).
..........
45min on hold
..........
Lady answers to tell me that there where no other flights leaving the UK. Which is a lie because id already checked and there are spaces for 10 of us to leave from bournemouth then she came out with the flight was cancelled because 'they expect bad weather'. The flight isnt for 3weeks!.
Obviously the deal we got when we initially booked was £65 return, now looking at £160 with another company.
Can I force them to stick us on the bournemouth or is it just a case of moaning and see if they give in?.