"There has been a steady drift in public mood against Brexit" - I'd wager this isn't true, which would resolve the apparent paradox. There may be disillusionment with post-Brexit UK, but that would be a different thing.

Both the UK and Europe are in measurable decline. Influential elements of the UK political and media class likely would like to reverse Brexit so that the UK's decline can be blamed on the EU again. All they would need would be for you not to notice the re-framing :-)