Thanks for your enquiry.
Nothing is legally binding until Contracts have been exchanged.
It would of course be normal for you to instruct a Solicitor to prepare the COntract/you would need Landlord's consent to the Assignment of the Lease and once this had all be done and both parties had signed their part of the COntract only then do you both have a legally binding agreement.
Until such time, either party is free to withdraw from the Sale and would not be responsible for the other parties fees or expenses.
I hope this assists and answers your question.