A SIM card purchased in the UK will work anywhere in the EU. That is not expected to change if the UK leaves on 29 March.
Some options:
https://www.lycamobile.co.uk/en/offe...yABEgI1-vD_BwE
http://www.three.co.uk/Store/SIM/Plans_for_phones
Lastly, there are Carphone Warehouses all over the place
https://www.carphonewarehouse.com/si...sort/relevance