With DAZN broadcasts that would make sense in order to boost subscriptions once they put their prices up but it wouldn't make sense to only do that to suit UK fans, it should be done to accomodate viewers in other countries too when one of their fighters is headlining and that would encourage fans to sign up.
But I can't see it for PPV broadcasters as they're likely to lose US market share if the main event is on at 3pm on a Saturday, people will be out shopping, taking the kids to their music lessons and running other errands and the casuals won't cancel those plans to watch boxing. The market share gained in say the UK won't make up for the loss of US viewership so it doesn't make any commercial sense. Showtime and ESPN are used to collecting $80 per household, British broadcasters certainly won't pay them that in rights fees.
And the last reason I can't see it happening is because the live gate will be compromised, fans will still turn up but they do more drinking and gambling in the evenings and those activities bring in the money for the venues.