As i do not no the vehicle in a technical way i cannot comment too deeply.
However, is this garage a decent garage or a backstreet garage?
the connectors for many items do not come seperately, i am highly suprised by the need for a new dash though.
can you afford the repair? and do they guarantee that the vehicle is now going to be fixed?
as the work has already begun i can only presume you have expressed you wish the work to commence.
the issue you have is that they will charge for any work already begun, but they cannot charge for the vehicle part unless vauxhall specifically say they cannot return the part and/or they have fitted it.
personally i'd probably allow them to repair the vehicle, ask to have all the old parts back, and for them to show you exactly what the fault was on the car.
But as the price of the part had gone up they legally have to ask you first before commencing further but can charge for any work before and after the price change to put the vehicle back to how it was.
so the best bet, is to let them fix it, pay and find out what the fault was. have a full reciept and warranty etc for that the fault has been fully fixed.
to take the car elsewhere is going to cost you more as they too will want to diagnose the issue.
next step is to find a family member or family friend who have a garage they trust and to use them in the future.
im sorry i cant be that much help but with limited knowledge this is the best i can offer. i have 7years experience in the trade so i can try my best.