I highly doubt Britain will be re-joining. The EU is headed for a bad place for a multitude of reasons (awful demographics, massive economic divergence between North and South, deep cultural splits between East and West, inability to reform in a logical way...) and will be a basket-case within a decade. I think the UK got out at a good time, will probably botch the Brexit process to an extent, but it won't be a disaster and the situation is recoverable.