This commentary arises from the decision of the CJEU in the case of Kaplan (C-77/19). The decision precedes the end of the Transition Period, and is thus binding precedent, save to the extent that this can be overridden by senior courts of the UK. This means, essentially, that the decision sets the applicable rule unless and until it is specifically revoked by a senior court or by a change in legislation.
The case involved an international supplies dimension (which the CJEU did not answer, since its answer to the other point rendered it otiose), and one that has application to entirely UK sited entities. The latter question fell into two parts, as follows: click here for more