Throughout Easter weekend London Euston station will be closed to all main line trains due to HS2 Engineering Work.
Avanti and LNWR Trains will start/terminate at Milton Keynes or even Crewe with road replacement transport to/from Bedford for East Midlands trains to St Pancras.
There is also engineering work between Crewe and Preston with trains diverted non stop via Manchester, extending journey times.
For Harry Potter at Watford only the all stations London Overground trains will run from Euston.