The London Overground (slow, all stations service) will not run from London Euston to Watford Junction between 3 and 8 August due to engineering work (the Bakerloo Line will also not run from Queens Park to Harrow and Wealdstone).
Any local passengers will have railway replacement buses provided north of Queen's Park to supplement normal London bus routes in the area.
This will not affect most visitors to the Harry Potter studios as they use the fast LNWR trains (they will run as normal), but something to be aware of. Avanti trains through Watford Junction will also run as normal.