A friend of mine last year signed and sent back a contract for a Turboprop out fit in Coventry. A week later got offered a 757 job. He just phoned the Chief Pilot up for the turboprop outfit and explained. They understood and simply ignored his signed contract. I think a company would rather have you working for them knowing that you have made the right choice.
I was like you until I got the reaction on the phone last month and now I think differently and will always look after number one.