Obviously, above a certain threshold (can’t remember exactly what…£70k-ish?), you have to be VAT registered, no choice and this can be on a rolling twelve month period.
If you’re selling mainly labour, and are underneath the threshold, then you can choose whether to be VAT registered or not. Most choose not to be for simplicity.
If you’re under the threshold but are making a lot of your money from supplying materials, eg carpet fitter, then it can make sense to be registered.
As neal says, a lot of people don’t understand this and think you’ve said “No, I’m not registered to pay tax.” when they ask how much VAT is going on the bill. 🙂