The first part of the story takes place in west London. Most of the locations named are genuine and can still be visited.
St Merryns Hospital, London
Bill is taken here to have his eyes attended to after being stung by a triffid. He is here during the night of the comet. There is no St Merryn's in London (which is a Cornish name) but its location is probably in Hammersmith or Fulham as Bill says when he leaves it:
To the left, through miles of suburban streets, lay the open country; to the right, the West End of London, with the City beyond.
It is probably also near the Thames because he mentions not being able to hear the "hoot of a tugboat".
The university building where Bill and Jo meet Beadley and the others, is now the University of London (Birkbeck College). The distinctive tower, called the Senate House, is still there although it is less prominent on the skyline than in 1951 when the novel was published.
Tynsham Manor, where the University party plan to set up their community and Miss Durrant eventually does, is located near Devizes in Wiltshire. Coming from London, Bill says:
Short of Devizes we pulled up once more to consult the map. A little farther on we turned down a side road to the right and drove into the village of Tynsham.
The village of Tynsham (which does not actually exist although there is an abandoned village called Tyneham in Dorset) is small and the Manor stands beyond it, surrounded by a high wall. It is within a landscaped park and is a large, rambling house built from many styles.
In reality, there is no building of that description in the Devizes area. The BBC used Sedgwick Park near Horsham (pictured above) in the 1981 adaptation.
Bill and Coker travel to Beaminster looking for Beadley and find Stephen Brennell and his party. The latter had been staying at Charcott Old House, a fortified manor house surrounded by a moat (not a real place).
Trivia: Beaminster is pronounced with a short E ("BEH-min-ster").
The farm that Bill, Jo and Susan eventually retreat to. It is on the Sussex Downs in the south of England:
"I know a lovely old farmhouse on the north side, looking right across towards Pulborough. It's not on the top of hills, but it's well up the side. There's a wind-pump for water, and I think they make their own electricity. It's all been converted and modernised." [Chapter 5]
The Sussex Downs are smooth, rolling chalk hills with wooded hillsides and uplands covered with grazing sod for sheep.
The building had originally been a farm but, over the previous 25 years, it had become a country house with turf laid over the farmyard (although the sheds and barns remained). It had its own well and power plant. It is about a mile or so (about two kilometers) from the nearest village.
In the 1981 BBC adaptation, they used a building in the correct area, on the road between Pulborough and Billingshurst (shown above).
Trivia: there is a character called "Shirning" in Wyndham's earlier novel "Stowaway to Mars".