It's not a UK thing, it's a company thing. For billable companies like legal and accounting, you are more likely to bill hours wherever you are. For most others

It's not a UK thing, it's a company thing. For billable companies like legal and accounting, you are more likely to bill hours wherever you are. For most others, it is "up to you" if you want to do work outside of paid hours but if you don't, maybe you won't keep up, maybe the boss won't like you etc.