It really depends on whether you think the NHS is enough for you. Opinion on the NHS is very varied. Maybe you should wait until you are here, see how you manage on the NHS, and if you are unhappy you can get private health insurance then.
On the NHS, you will never have to worry about seeing a doctor in an emergency or seeing a doctor if you get sick. As has been said, private insurance is more for specialist care, and some people are happy with the specialist care under the NHS and some aren't, so it depends on your personal situation and wants.
If you are going to get private health insurance, there are a number of different companies so you shold shop around for the best deal for you.