It really isn't enforceable ... I used to work for a very large commercial law firm and even though the HR department wanted to use clauses like that the lawyers told them they couldn't.
What can they do if after you leave? Put you through a disciplinary, you've left so they can't do that. Withold payments you are due? No, you have a left so nothing to pay you and if they do that is way illegal so can't do that either. I can't even see how they can sue you for breach of contract, you leave them on Friday and start new place on Monday. Whilst working for them you weren't working for anyone else, and then on the Monday the contract has been terminated so no longer applies.
A former employer cannot dictate who / where you work for. These clauses are put in to scare people into thinking they cannot move or work for anyone else, I really would just sign it and forget about it.
Mat