In the UK is it illegal to have fog lamps on in good weather?

Yes, it is illegal to have fog lamps on in good weather in the UK.

According to the Road Vehicles Lighting Regulations 1989, fog lamps may only be used "when visibility is seriously reduced". This means that they should not be used in good weather, as they can dazzle other road users and make it more difficult for them to see.

If you are caught driving with your fog lamps on in good weather, you could be fined up to £1,000. You may also be given points on your licence, which could lead to a driving ban.

So, if you're not sure whether or not it's safe to use your fog lamps, err on the side of caution and turn them off. It's not worth the risk of getting a fine or points on your licence.