During the pub scene Dani's hair and Henry's pint level change depending on the camera angle.
In the pub, the amount of hair in front of Dani's left shoulder changes notably between shots.
The telephone in the kitchen of Bly Manor rings with a North American ring...ring cadence, not a UK ring-ring... ring-ring cadence.
The phones in the manor are all Western Bell models. Common in the USA at the time, but not used in the UK.
At the beginning of S1E1 the young teacher is filmed on a London Street, caption "London 1987".
She's standing in front of a Richard Mille window. Richard Mille was founded in 1999.
The flora and the architecture suggest the US, rather than England. The 'London bar' simply does not look (or sound) like the real thing. Essex, where 'Bly Manor' is situated, is portrayed as if it is deep in the countryside; in fact, it is adjacent to London.