https://www.captiveportal.co.uk works and doesnt redirect to a non-https link, at this point its hard for normal users to go to an http:// link without their browser overriding them
neverssl.com solves this by redirecting to a random subdomain (for some reason that isnt clear to me near midnight)
a .co.uk equivalent is a great idea though, if it can be made accessible to users with hostile browsers