This is a relatively straightforward procedure.
The aircraft needs to be of a type approved by the CAA, and needs to have been built and maintained in accordance with the type certificate (not difficult in most types). All CAA ADs need to be complied with (in keeping with the planned Certificate of Registration type) and a Star Annual needs to be performed.
The aircraft will be inspected by a CAA Surveyor, a fee will be paid and a C of A issued.
The marginal costs over a normal Star Annual include the Surveyors visit (not a lot from memory) and the fitting of such CAA delights as the LV light.
Overall, it is much cheaper/easier to do it in this direction, than it is to put a G back onto the N.
2D