The 1-day tickets to Universals in America are priced to incentivise longer trips; the tickets with more days on them, such as the 14-day 3 Park Explorer Ticket at Universal Orlando, are £339 per adult (10+) and £329 per child aged 3-9 in 2024:
https://ukstore.universalorlando.com/park-tickets
This works out vastly, vastly cheaper per day. And that’s before you consider that those tickets are probably pricier due to them letting you into multiple parks; the 14-day tickets now give you unlimited entry into both theme parks and Volcano Bay for 14 days.
I’d imagine that any Universal offering in Britain would probably be similar, with multi-day tickets working out cheaper on a per-day basis.
Another thing to bear in mind in terms of Universal vs Merlin pricing is that the Merlin parks, particularly the parks in the South, are filled with a considerable army of frugal passholders, who buy MAPs for £100 or so each at the start of the season, take a packed lunch and go and get their money’s worth at the Merlin attractions every weekend. You can bet your bottom dollar that Universal’s annual passes will not be nearly as cheap as those sold by Merlin, which would likely price out many of the people who buy MAPs or similar.