Surfing the internet, I found the EASA is ready to approve as many as 233 existing British Maintenance Organisations in case of no deal Brexit.

It will certainly smooth the transition until, still under such a Brexit scenario, an agreement like the one between EASA and Switzerland or EASA and Norway, will be signed with the UK as well (an agreement with countries not belonging to the European Union).

Here is the List (original link at the EASA website)

If of your interest, I truly recommend to check the list with regularity, as it updated by the EASA on regular basis.