In the UK, your contract is formed by: the thing you signed, any statutory law stuff, and any custom and practice stuff in your workplace. Generally speaking however, it's the same as what other people in this subthread have pointed out: you probably can't do a lot if the employer changes your contract, either officially or unofficially. Continuing to go to work implies acceptance and well. You could attempt to take the employer to an employment tribunal,but they're expensive (even more expensive thanks to the recent Conservative governments)

Source: used to be a trade union official. If there was a union for tech jobs, they might be able to pay for tribunal costs, but when I did it we only took them on if there looked to be a greater than fifty percent chance of winning.