Can that apply if the OP resigned without making a case for constructive dismissal?
OP, I believe the law states that when you enter into a contract that Ts&Cs need to be agreeable to both parties. However, some of these may be against the law. For example, I could make you sign a contract that states that you will sell me your firstborn. It maybe agreeable to you and me, but it’s illegal and if you wanted out then no court would make you sell the child to me.
I’d ask them to clarify exactly which things they feel make it justified for them to ask you not to seek employment. The burden of proof to show that you seeking employment will threaten their legitimate business interests and is of a period of suitable length falls on them if they want to go to court anyway.
For example, their case will be stronger if you are a senior employee, or are party to trade secrets.
Your case will be stronger if you are not a senior employee, or are moving to another geographical region.
Ultimately, get your solicitor to write asking them why it’s justified and to prove it and they may decide to let it go. Remember, lawyers aren’t expensive, it’s going to court that is. A good one will usually try to get you to settle before it goes to court.
Good luck.