A few years ago, when I was in recruiting, I attended a legal seminar on restrictive covenants because at the time it was good to know the legal framework and so on around.
The general theme is that the clearer the time and scope limitations are, the more likely they are to be upheld – i.e. it’s probably OK to say that customer-facing staff can’t work for customers they’ve served in a similar role for a few months, but not to say they can’t work in a similar role for a competitor for a year.
Six months is generally seen as a sort of cut-off point, but there’s no hard and fast rule – it depends on which judge is presiding on the day. Also, you’ll have to pay costs for a High Court injunction to enforce such a clause, so budget in a minimum of £10,000 which you’re unlikely to get back.
In the case of redundancies, it is highly unlikely you will be able to enforce anything. Courts don’t like to restrict someone’s ability to earn a living. You are saying “we don’t need the position, but we want to stop the person who fills it from trying to find the same job elsewhere”, which is not very reasonable (or fair). Remember that it is the role that does the redundancy, not the person who does the redundancy.