Bank of England
We do not exchange coins
Please contact the Royal Mint for information about coins or to exchange coins
The Royal Mint
How can I dispose of coins no longer in circulation?
It is our understanding that some United Kingdom high-street banks are willing to accept demonetised coins from their customers. Please be aware, however, they are under no legal obligation to do so.
That seems to suggest that your coin is no longer a legal tender and a monetary exchange is not guaranteed.
I would keep it as a souvenir.
However, if you still like to sell your coin this may help:
Does the Royal Mint buy coins and medals from the general public?
The Royal Mint does not currently purchase old coins or medals.
If you wish to sell your coins we would recommend that you contact a reputable coin dealer such as Sovereign Rarities Ltd. www.sovr.co.uk or your local BNTA member www.bnta.net
A very relevant news article: How to swap your old £1 coin for a new one at your bank – when did they change and how long will banks accept the round pounds?