Roman Emperor Tacitus, ruled late 275 - mid 276; last emperor to be elected by Senate vote, but died after nine months from illness. He briefly reversed the trend toward the emperor ruling openly as a monarch, and won a minor campaign against the barbarians; otherwise his reign is unremarkable and few exept Roman collectors or historians could tell you anything about him. He is scarcer than most emperors from the third century, but not rare by any means.
£14 is about right - it would go for between $20-30 on ebay. Do NOT damage the coin or put it into jewelry! This is a very collectible coin.