As with most business that envolves customers you are only ever buying goodwill,but with good contracts and well established customers it could be a good buy for some one.
How much would you be asking for if it was your work?
Your not going to pick up quality commercial window cleaning work for 3-4x the value.
A large commercial cleaning and window cleaning company sold were I live a few years back for millions( and I do mean millions)take away a few assets and staff and all they bought was contracts,but it still sold and is being worked now.
One thing with work in London is running costs and travel time parking meters and congestion charge will cost a bit each day.