Nothing to do with Brexit, banks have been reducing their high street presence for 20yrs plus in the UK.

Santander had some banks become part Costa Coffee and part bank to reduce costs as another example of money saving.

Too expensive to keep branches open

Nobody really does physical banking any more so the banks are closing their physical branches. There's a lot of backlash because a few people can't use online banking, it's difficult because the numbers are small and it's hard to justify staffing a full building when 5 customers turn up each day.

It's not brexit related.

Ah, OK thanks for the insight

When you think about it, it does make sense from the business point of view. Banks don't make money on individual customers, so the ones that use physical locations. They make money off corporate customers, who are terribly unlikely to ever need to talk to a clerk in person. And with individual customers moving to online banking, it's even less justifiable to run a physical location.