(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.
No, they can only exercise the privileges in an Annex 1 aircraft. It has not been legal to fly an EASA aircraft on a UK National Licemce since 8th June 2018. The UK could issue an exemption after 31st December.