Don't bin that coin… yet.
The value is subjective.
Without the countermark, the coin is worth less than one euro to a collector. In fact, some collectors abhor alterations like this and would have no interest in the coin st all. However, other collectors search for countermarks like these and pay a premium for them. The problem is “provenance”. It is difficult, if not impossible, to prove who marked the coin, when, or why. King is probably right, the mark likely refers to “Ulster Volunteer Force.” Whether the mark is a recent alteration, no one can say. Thus, the value is even more subjective and it might be difficult to find the right buyer for this coin. If you list the coin on ebay you might achieve 5 or 10 euro on the high side, not worth the effort to me.
But don't bin the coin! It might not be worth a lot of money, but it will be of historic interest to someone. If you don't manage a sale on ebay, hold the coin for your collection, you might find a use later on.
BTW: read these links for related info:
https://www.coinbooks.org/v22/esylum_v22n40a19.html
https://www.royalmintmuseum.org.uk/podcast/mark/