Answer:
Immediately after Brexit day - on 31 January - the UK went into a transition period with the EU. This was to give both sides breathing space to agree the new arrangements.
During transition the UK is still following EU rules and trading in the same way as before. However, the UK is no longer part of the EU's political institutions - so there are now no British MEPs in the European Parliament.
This transition period will last until 31 December 2020, when the new EU-UK relationship will begin. The deadline for extending the transition has now passed.