Surely...
(a) A UK CPL licence can be used to fly any G-reg aeroplane for which the pilot is suitably qualified, within the privileges of that licence. Hence a UK-FCL CPL or PPL in, say, a G-reg PA28 has IMC rating privileges in UK airspace.
(b) All UK licences remain ICAO compliant.
So, notwithstanding Fl1ingfrog's Eurosceptic stance about EASA's attitudes above (with which I substantially agree) the UK licences will be in broadly the same place as any other ICAO contracting state's, such as e.g. an FAA licence.
What will doubtless be painful is EASA's clear intention to refuse to accept mutuality of the UK's substantially identical aviation regulations. This is part of the overall bullying attitude of the EU towards all things British, as it's so important to make Brexit an abject failure, so as to dissuade all others. Sooner or later somebody will probably test this in court, which will be interesting.
G