The idea is not realistic. It is not supported by any major political party in the UK (apart from the Scottish independence SNP). Labor does not even dare to mention the return to the European internal market, that is to say. Brexit has divided the country too much, the British don’t want to hear about it anymore, they want to “move on”.
After years of psychodrama, thousands of hours of negotiations, the Europeans have other priorities. And they certainly do not want to relive, “in reverse”, a process of integration with a neighbor, unless the latter shows, over several parliamentary cycles, a very clear desire, and by a large majority, to return to the circle of the European Union.