Hi, OP here, good to see the overwhelming response is that IA NOT BU!
Started the job this year, worked through the lockdown. I get 20 days holiday and it's my first holiday since I started. I asked should put my out of office on and was told 'No, we need to be available for our customers'. Boss owns the company, so obviously they are available 24/7.
Possibly because I'm staying at home and not 'away', the boss thinks this is a reasonable request 
Ironically, I have a work phone but emails aren't accessible, and was told 'we don't have emails on phones, as you get interrupted at home'.
I will probably do it this time as the job is hugely convenient and given the economic situation atm I am lucky to be employed but next time I'm off I will be away somewhere remote without internet access! 
My work phone will be switched off, as it always is at weekends when I'm off.
I just feel pissed off because I always go over and above when I'm at work because that's the type of person I am but when I'm off, I expect to be able to switch off from work properly.