The size is something between a 1p and 2p coin (£pound sterling)

http://www.medievalcoinage.com/saurma/someoldcoins.org/saur/g/s336.htm

I believe the title is in German but that appears to be your coin. Looks like the dates range from early 1400-1500. I'll do more digging

The cross looks like that of the Teutonic Order. Not sure how that ended up in the UK, would have likely come from the areas between Germany and Lithuania.

Livonia refers to an area along the Baltic Sea. There's a map of it here https://en.wikipedia.org/wiki/Livonia

I'd put the latest possible date on the coin to be 1561, the end of the Livonian Order.

The condition of that coin seems to be to good be an original from that date however