As of 2020, the oldest circulating coins in the UK are the 1p and 2p copper coins introduced in 1971. No other coins from before 1982 are in circulation. Prior to the withdrawal from circulation in 1992, the oldest circulating coins usually dated from 1947: although older coins were still legal tender, inflation meant that their silver content was worth more than their face value, so they tended to be removed from circulation and hoarded. Before decimalisation in 1971, a handful of change might have contained coins over 100 years old, bearing any of five monarchs’ heads, especially in the copper coins.
There is a continuous history for the GBP from the 800s up to now, but that doesn’t make it legal tender. The good news is that they’re all worth way more simply because the value of silver is higher than the face value of the coin.