There will be industrial action by members of the TSSA union on LNWR and West Midlands Trains on 10 and 11 July- which will lead to significantly reduced services on all their routes.
See here for details- https://www.nationalrail.co.uk/service-disruptions/industrial-action-wmt-20260703/
This will affect travel to Watford Junction for Harry Potter and to Bletchley Park, where times will need to be checked. It may also lead to overcrowding on the trains which do run. On Friday especially all services will end by 1900. But the London Overground will still run, as will the London buses routes to Watford Junction.
For Bletchley another option is Avanti non stop to Milton Keynes then local buses
The notice period for industrial action has recently been reduced from 14 to 10 days, under the Employment Rights Act 2025, then the train company did not release notice to the public for a further 3 days.