How can a verbal agreement be enforced (UK) once it has been broken by the other party, in this case an agreement to purchase a boat and given four days to make payment but the boat was sold to another party in the interim

A verbal agreement is generally NOT worth the paper its printed on. With no supporting documentation or witnesses, its would be hard to prove it existed.

Plus, in this case, what exactly would the case be? You are out a boat, but it doesn't appear that you are out any money. And it's not like the seller can go get the boat back from the person who did buy it and give it to you.

If you put down a deposit, then you should be able to get that back.

A verbal agreement is generally NOT worth the paper its printed on. With no supporting documentation or witnesses, its would be hard to prove it existed.

The great old Louie B. Mayer quote.