Financially, Britain cannot be a geopolitical super-power without its now gone empire.
Had we kept Hong Kong, it would have bankrupted us trying to keep up with the defense spend needed to keep China at bay.
Plus, we don't have the oil/gas, farmland, minerals and precious metals to fall back onto either to be at least somewhat self sustainable to some degree. Russia and the US do. If Britain was to have sanctions put on it we would be utterly screwed.
Britain needs to focus on trying to be a beacon of education and business/finance sector, a Singapore of some sort whilst having a military on standby if needed in the same way Norway has.
We are chicken feed on the world stage nowadays. If you took London's economy away from the UK, it would be poorer in GDP terms than the poorest state in the US, Mississippi.
I am not particularly for it but I do think by 2050 we will have a Federalist Europe with Britain part of it.