Is it legal to go metal detecting in the countryside?

Depends on your country,

in the UK you have to have permission off the land owner, all land has an owner, whether it be the government, the crown, or private such as farmers or national heritage organisations.

protected sites are not allowed, so you need to do research on a site first, this just about rules out protected sites such sa castles, roman settlements and such like

bottom line, find out the owner, get written permission