My rather basic understanding is that the Succession Regulation does not apply to the UK.
However, an English will can include a choice of law clause which for an English national would be English law which would then have universal application (Art 20). This would mean that for succession purposes the Member States would then apply English internal law (no renvoi ?; Art 34(2)).
Whether the UK leaves the EU after Brexit (or not) should not affect the choice of law clause as to make such an election does not require the UK to be a Member State.
The consequences of the Regulation would seem to be that an English national (UK domiciliary) who is habitually resident in a Member State or has property located in a Member State ought to include an English choice of law clause in their will. This will then ensure the application of English law.
Presumably, to the extent that land is located in a Member State English law would then apply to the succession (as intended by the testator) subject only to any conflicting issues under the law of the territory where the land is situated (eg so-called “foreign heirship” provisions) in which case the law of the latter would apply.
Should existing wills be reviewed/rewritten so as to ensure a choice of law clause is included for an English national residing in or with property in a Member State (or can reliance be placed on the transitional provisions; Art 83)?