The issue seems to be whether a contract has been concluded or not.
S.17 (1) of the Sale of Goods Act 1979 states that "where there is a contract for the sale of specific...goods, the property in them is transferred to the buyer at such time as the parties to the contract intend it to be transferred".
S17 (2) goes on to state that "for the purpose of ascertaining the intention of the parties regard shall be had to the terms of the contract, the conduct of the parties and the circumstances of the case.
In that context it seems to me that by completing both sets of registration documents, your conduct shows that you have entered into an agreement. What other evidence is there though? Is there anything that shows evidence of an agreed price?
My view is that property has passed and it may be that the Court would have to decide on any further matters such as what amount of money should change hands if either party 'forgot' what was verbally agreed. I may have a solution though.