I didn't realise this was back (semi)working
http://www.old-maps.co.uk/
maybe it has for ages.

And this is rather interesting (though not terribly megalithic)- a map of london from 1827. eg Earl's Court all surrounded by fields, New Cross just a tiny crossroads, Euston with no station, etc.
http://users.bathspa.ac.uk/greenwood/