It's more complex than that.

Firstly, the UK is three separate legal jurisdictions, each with their own rules on metal detecting: England/Wales, Scotland, Northern Ireland.

Secondly, these rules are not binary legal/illegal, but on a continuum of permissiveness. You always need some kind of permission. England/Wales is more permissive (where most non-protected land can be detected on with the landowner's permission) than the Republic of Ireland (where you need state approval to use a metal detector anywhere).

Some of the details here: https://detecthistory.com/metal-detecting/uk/