Then they can influence the laws that guide their policies as a European Union instead of just cutting off their ties with the EU in what they referred to as the British Exit or Brexit. In a world of economic uncertainties and insecurity, it is rather better to get into ties than to get off ties with any union right now. I didn't encourage the UK to cut off for once but they did what they have to do and it's already backfiring on them because it wasn't going to be easy to break a mountain that was built for over half a century which the UK must have also enjoyed too, in just few months or years.
as you said, the uk got various benefits when they joined the european union, but the problem was when the benefits they got were not commensurate with what they had to endure, and that was what pushed them to leave the european union and become a completely independent country. even though it will have an effect on the uk economy, at least it will make them understand more about the problems that exist in their country and will slowly improve them through targeted policies.
What benefits?
Name one that we havent had to pay for in excess.