cost of brexit is about 2-3% of GDP roughly the same as spending on immigrants. (most of whom pay taxes.)
also worth mentioning is that the aging UK population would be in an even worse state without those workers.
leaving the EU is a perfect symptom of the other problems you listed.
https://cepr.org/voxeu/columns/impact-brexit- uk-economy-reviewing-evidence
https://migrationobservatory.ox.ac.uk/resources/briefings/th...