Sort of the minimum price. A contract is an exchange, therefore for it to be valid the 'purchaser' must offer something of value. This is often written as a 'Peppercorn' or 'Nominal sum' but £1 is often used where a monetary value must be placed. In practice the £1 rarely changes hands. Using a defined monetary value also limits the potential risk of legal action at a later date.I've heard a few times that upon privatisation the Waterloo & City was sold from BR to LT for £1 (allegedly the minimum price of a contract under English law), but have never actually been able to find a source confirming this.
Plenty of pieces of infrastructure and rolling stock on preserved railways that are normally considered 'donated' were in fact purchased for £1.