The UK is slightly odd in that anybody can choose to fly IFR in VMC it's just a set of rules. To fly in IMC you have to fly IFR and in Class G that implies quadrantal levels (the rest of the world uses semi-circular).
The IMCR allows IFR flight in the UK in Class G and Class D (BCEF are unlikely to be relevant) but not Class A where special VFR is OK down to 3km vis which are lower than the normal UK minima of 10km.
An IMCR holder cannot therefore fly airways in England but there are Class D airways in Scotland where you can.
Consequently filing an IFR flight plan for an IMCR holder is generally pointless.
To return to the original scenario of a flight along the South coast to Exeter from Southend:
You depart VFR and happily chug along at whatever altitude you want, clear of CAS, lets say via MAY and GWC. (Though you might get a transit of Gatwick if you ask!) Approaching GWC you enter cloud so adjust to the correct quadrantal for heading and call Solent "GABCD Request zone transit and traffic service" They'll ask for details and you give them including flight rules IFR.
They'll hopefully give you service and a squawk and a transit then hand you over to Bournemouth who also give you transit and service and then hand you over to Exeter. If busy they might invite you to freecall the next unit.
Job done really.