The story 'Dalek's Invasion of Earth' had extensive location work across quite a large area including the centre of London and docklands. Such locations in central London are largely unchanged and while I only took a few pictures, there is in fact a whole website dedicated to the subject but there's nothing quite like seeing a place for yourself.
I followed the sequence from episode 3 where Barbara, Jenny and Dortmun cross central London across Westminster Bridge, through Whitehall to Trafalgar Square. As one of the first stories to make extensive use of location work, they made efforts to show it off as much as possible!
The first part is on the south side of Westminster bridge. In the background there's some Dalek writing, or maybe it's Dalek graffiti which appears in several places - the vandals! Here they have preserved the wood or metal blocking the gateway by writing on a sign and attaching it. When it comes to famous monuments they write directly on it...
For a moment the group hold back, as Daleks pass over the bridge...
But then have to hurry onwards as a Dalek arrives at the bottom of the steps with Westminster clear in the background.
The group make it over the bridge and we quickly rejoin them in Whitehall. We don't see Downing Street but they do take shelter for a moment behind the statue of Prince George. Again with the Dalek Graffiti! Would the BBC get away with painting on a famous statue today?!
Upon arrival in Trafalgar Square the place is swarming with Daleks. Several shots were filmed around this area... Trafalgar Square also appeared in the montage of shots at the end of 'The Chase' where Ian and Barbara celebrate returning to Earth.
Finally a Dalek is seen coming around the corner of the Charles James Napier statue, after this the group make their way up past the National Gallery.
The Dalek graffiti is not something I've seen discussed much, but it is surely one of the few examples of their written tongue. Maybe it's numbers rather than a word or a name, who knows?
The BBC have to be commended on managing to film all these sequences without any people or traffic around. Likely early morning work, but it would be impossible today unless you filmed on Christmas Day, the city is just continuously busy and they are unlikely to close and clear such an area for a TV programme.
Also the production team actually painted on the statues, the graffiti is effectively genuine. This would never happen today, programme makers just wouldn't do this for fear of fines and complaints. Back then it was just a guy (designer Spencer Chapman supposedly) with some enamel paint, and even he annoyed some authorities according to one site I have read.