Re: What's the point of a "contract"
They are not changing in the middle without recourse. They are exercising the termination clause in the existing contract. They are then offering a new contract at worse rates. That's one of the risks of contracting and the the contractors are free to refuse the new contract or to try and negotiate alternate terms.
People reguarly do the same thing with mobile phone "contracts". Once they are out of the lock in period they can terminate the old contract in favor of a new contract with "better" terms, aka more free minutes, texts, data. It's exactly the same thing.
To the "contractors" at BT "4 weeks is not a contract, it's employment. Most contracts have at most a 2 week break clause. Typically 1 week, sometimes less."
Disclaimer: I'm a contractor (not with BT) and I have been contracting for many years.