I'm afraid that it does, as far as I know. I know of someone who did something similar and was taken to the High Court over it, resulting in huge expense, probably mainly to 'set an example'. The only possible get-out might be an argument that "12 months" is too long a period and is anti-competitive - 6 months might be more reasonable. However, it could well be expensive proving it. In the above case, there was no period specified, and it still cost an awful, awful lot of money and stress.
I think perhaps your best options might be to either push your current agency for a higher rate, or else to just bite the bullet and move job completely.