who will automatically receive a UK set of licences on 1st January 2021,
You are not likely to "receive" anything new, your existing EASA Licence which is issued nationally in accordance with EASA standards remains a UK licence. Not shure how that will differ from the earlier pre EASA UK licence that many of us still hold.
To teach at an EASA approved ATO you will need to comply with FCL900(c).
(c) Instruction provided outside the territory of the Member States:
(1) By way of derogation from point (a), in the case of flight instruction provided during a training course approved in accordance with this Annex outside the territory for which Member States are responsible under the Chicago Convention, the competent authority shall issue an instructor certificate to applicants who:
(i) holds a pilot licence that meets all of the following criteria:
(A) it complies with Annex 1 to the Chicago Convention;
(B) in any case, it is at least a CPL in the relevant aircraft category with a relevant rating or certificate;
(ii) complies with the requirements established in this Subpart for the issue of the relevant instructor certificate;
(iii) demonstrates to the competent authority an adequate level of knowledge of European aviation safety rules to be able to exercise instructional privileges in accordance with this Annex.
(2) The certificate shall be limited to providing flight instruction during a training course approved in accordance with this Annex which meets all of the following conditions:
(i) it is provided outside the territory for which Member States are responsible under the Chicago Convention;
(ii) it is provided to student pilots who have sufficient knowledge of the language in which flight instruction is provided.