With the extension of the Brexit deadline, the British should still be part of the EU when the new Commission takes office. The treaties provide for the Commission to be composed of one commissioner per member state.

It is an institutional puzzle to be sorted out for the next President of the European Commission. The British government told Ursula von der Leyen on Thursday 14 November that it would not propose a commissioner to join his team before the December 12 elections in the UK.
Concurrently with this letter sent by the British government, the next president of the European Commission has obtained a legal opinion that the absence of a British commissioner will not prevent his new team from taking office.
Brexit pushed back
Initially, the treaties provide for the Commission to be composed of one commissioner per member state. However, with the extension of the deadline for the Brexit (passed from 31 October 2019 to 31 January 2020) granted recently, the British should still be part of the European Union (EU) at the time of entry into office of the new Commission. This is in fact planned for 1st December, instead of 1st November initially planned, as three candidates for the new team (nominated by France, Romania and Hungary) were rejected by the European Parliament. Three new names must be proposed for approval by the European Parliament.
This absence of a Commissioner from a Member State is not a first. The previous commission was already working with 26 members, instead of 28, since the departure of the Romanian and Estonian commissioners for the European Parliament. These two Member States had left, at the request of Jean-Claude Juncker, the two vacant seats for the remaining few months.