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Dear Client, my name is***** am a solicitor. Please be advised, that formulae involve undertakings by solicitor, that is why they are not suitable for use if one party is unrepresented. Hope this helps.
You may do the personal exchange, please ask the other party's conveyancers if they would agree. If yes, you will go to the sellers' conveyancers' office to do the exchange in person. Alternatively you could send them your copy of the contract and cheque for the deposit, and ask them to hold these to your order until you agree an exchange with them over the phone.
If they agreed to have no undertaking from you to send them the signed contract after the phone exchange, that is fine. Formula B is used when each party hold its copy of the contract. If you held your copy and the buyer held its copy, then it is formula B.
Sometimes one party's solicitors hold both parties' contracts, this is formula A. Formula C when there is a chain of transactions.