The answer to your question is how much they are will pay.

Also you're in a curious position because if they have filed a Trademark on their name (in US at least not sure about UK) they can file a trademark dispute.

If you came to me and said you were squating on my TM I'd start legal procedures.

Now if they don't have a TM ... Maybe convince them to do a profit share if this new domain rockets them to new search levels. Exact domains have a much higher SEO value. So in truth it could be a win for them. I'd reach out to who ever does their SEO and let he/she be your advocate.