How can I bind a UK seller to the sale before exchange?

Hi, let's say I've had an offer accepted on a property I want to buy.
However there is a local opportunity which the seller may discover (as I
did) if he talks to the right people.

I want to legally bind him to sell to me at this early stage of having
accepted the offer verbally and by e-mail.

Is there such a standard document (in the UK) which will allow this and
which someone would normally expect to sign?

Any other things that could be used instead, to hold them to the agreement.

Thanks :)