Hello and thanks for using Just Answer.
I'm Al and hopefully can assist you with your enquiry.
Even though your husband has left you all his Estate, the Land Registry will require evidence that you are legally entitled to deal with the land,as his Executor. Probate is the only evidence to prove you are entitled to sell the land and therefore the Buyer's Solicitor is quite correct in requesting it, I'm afraid.
I hope this assists and sets out the legal position.
However, pleas elet me know if you require any further clarification.