Money transfers between bank accounts for no extra fees (well, that's limited to the Euro, so a couple of countries chose to stay outside). Mobile phone usage for no extra fees all over EU (some limitations, but typically not relevant for the average trip).
B2B the integration has even bigger impact.
I don't think eletric plugs are even regulated. British plugs never changed and they were not an argument in Brexit.
My bank requires me to fill in a paper form, scan it, send it by mail, wait a week, then get an answer (often, "no, give us more details") if I want to send money in another country. Neobanks tried to solve this for lower amounts, but will freeze easily your account for "AML" if you send low six figures.
B2B the integration has even bigger impact.
I don't think eletric plugs are even regulated. British plugs never changed and they were not an argument in Brexit.