Indeed. 'Impose' almost suggested they were being forced to sign it. The thing is junior doctors are among one of the few professions that are in demand pretty much everywhere. The government is proceeding from the industrial relations angle they normally use - "Sign the contract. It's either that or the dole. Do you want to live on 75 quid a week? Do you?" However your junior doctor can simply reply "Really? screw you. I'm off to Oz for beaches & barbeques and more money". Heck if you in the midlands or north west you can commute to hospitals in Wales and not have to deal with it. This government simply can't seem to understand the notion that it doesn't have all the power in an industrial relationship.
Apparently all junior doctor contracts are a year long and they have big game of musical chairs every August, been like that for years. Seems quite inefficient to me. The trusts do have the right to offer whatever contract they like, after all they're technically private businesses.
You're assuming credibility is a requirement for a minister in this government.