To stay out. The UK left because it refused to adapt its immigration policies to EU dictates, which were mostly German dictates, and this area of conflict with the EU will not change. Brexit was harmful to the British economy because the British government naively thought it could negotiate trade deals with the EU that would allow it to continue profitable exports to EU markets without having to adapt to EU, mostly German, rules. The solution to this problem is to work harder to sign bilateral trade deals with nations outside of the EU.