This was LAA reply to my enquiry of the same:-
This email is to let you know that with the impending finish of the Brexit transition period, the CAA have issued a general permission ORS4 number 1457 which allows G-registered aircraft such as your GY80 Horizon that operate on the basis of an EASA Permit to Fly to continue to operate within UK airspace until the end of 2022 despite the UK having withdrawn from EASA participation. We will let you know in due course whether there are any changes required to the arrangements for revalidating the Permit when it falls due when your ‘annual’ comes up in the next 12 months, or whether this can take place in the normal way. We will also inform you of any changes to the procedures for approval of modifications and repairs, operation or maintenance.
The situation with regard to flying outside of UK airspace with a G-registered EASA Permit aircraft is not yet clear, the arrangements covering mutual acceptance of G- registered aircraft with a full ICAO CofA does not extend to Permit to Fly aircraft. Following the release of the details of the 1800 page Brexit deal document on Christmas Eve we are in discussion with CAA about this issue and have already received positive feedback from the DGAC about overflight of French airspace.
The general permission is a temporary measure to allow an efficient and unhurried transition to whatever permanent arrangements are put in place. LAA will continue to work with the CAA to ensure a satisfactory long term outcome.
A copy of the CAA ORS4 number 1457 can be found here: https://publicapps.caa.co.uk/docs/33/ORS4No1457.pdf
Before the next flight after 1st January 2021, it’s important that you read this document and only fly the aircraft if it has a copy of the document on board, as well as, of course, the aircraft’s EASA Permit to Fly being valid and having a current Certificate of Validity. The CAA general permission must be used in conjunction with a valid EASA Permit to Fly, it does not replace it.
Enjoy everything - Life is Short.