It is not illegal to tie something to the roof of your car in the UK. However, there are some safety concerns that you should be aware of before doing so.
For example, if you tie a large object to the roof of your car it could block your view when driving and make it more difficult to see other vehicles or pedestrians. This could increase the risk of an accident happening.
It is also important to make sure that any objects that you tie to the roof of your car are secure and will not come loose while you are driving. If an object fell off the roof of your car and hit another vehicle or pedestrian this could cause serious injury or even death.
So while it is not illegal to tie something to the roof of your car in the UK, you should consider the potential risks before doing so.