A previous owner of my boat went from husband and wife to just husband, presumably on their divorce: there was a MCA (Pt 1 Registered) Bill of Sale from Mr and Mrs to just Mr. I'll have a look at it tomorrow.
As I understand it - the £1 nominal sum use came about many years ago when items needed to be transferred and incur minimal legal asset value. The old days of the BoS with a Postage Stamp and £1 plus signature across the stamp has long been a matter of history.
As I understand it - from my own experience in UK divorce .. there was not any requirement to state a 'sale' value as long as both parties agreed and signed a Transfer Document - stating clearly the transfer and the one party no longer had claim to the item.
A previous sale of my boat was valued at '£1 and other considerations'. The man I bought it from was a car dealer, who told me that the 'consideration' was a Mercedes car.
However this is irrelevant now for the purposes of a Bill of Sale which is just the transfer of title but as in post#24 the reasons for including it in the standard form is historic. However as already noted, other documents are equally valid as evidence of title - it is just convenient to use the MCA one which is required to register the title, even though the vast majority of leisure boats do not have registered title.