I don’t think you can play on demo without an email sign up any more. Sky Vegas did let you do this until around 2017 then a UK gov loophole got closed where you had to be signed into Sky Vegas in order to play on demo.
I’m fine with logging in with an email address as it’s just a few clicks and no password is needed.
https://www.nolimitcity.com/games – Nolimit City
https://play.bigtimevegas.com – Big Time Gaming
A VPN is needed if you want to demo play bonus buys on these sites.