Very much outdated but £400.
The most expensive property on the UK Edition of Monopoly is Mayfair, costing £400
£400
mayfair
Mayfair
300
Mayfair is the most expensive property in British Monopoly.
Boardwalk
Mayfair (not Mayfair Street) then Regent Street & finally Trafalgar Square.
The Monopoly edition you're referring to is the "Monopoly: The Classic Edition," which was released in the UK in 2000. This version featured the standard UK board with Mayfair and included pink Community Chest and blue Chance cards. All the properties were represented with dollar amounts using white notes, ranging from $1 to $500.
Mayfair
Mayfair
Pink!