When Covid first hit, National Express greatly reduced their services with many routes being 'consolidated' so that, for example, to get to Airport A you now have to go via Airport B. (Similarly, to get into central London, passengers might need to go via both Airport A and Airport B en route). In order to keep journey times down for those longer routes, lesser-used stops were removed from the timetables altogether.
So it is with the Worthing-Brighton-London routes, where all of the various services that previously operated (into Gatwick, Heathrow and Victoria Coach Station) have been consolidated into Service 205. That service now runs from Worthing and Brighton to Victoria Coach station but going via Gatwick and Heathrow to get there (and missing out Hickstead altogether).
If you want to use Service 025, you'll have to start out in the wrong direction, by taking a bus or train to Brighton. Service 025 stops in Brighton at Pool Valley, Preston Circus, Preston Park, Withdean Park and Patcham. Due to having to go via Gatwick, the journey to Heathrow takes about two and a quarter hours:
https://timetables.nationalexpress.com/routes/NX/025/Worthing-London/I/pdf?from=65&to=471
To get to Brighton in the first place from the Hickstead area, you'd need to use the Metrobus 273 service:
https://passenger-line-assets.s3.eu-west-1.amazonaws.com/metrobus/MB/273-timetable-20210828-e50b0327.pdf
In order to be able to advise you on any possible options using rail services (either exclusively or in combination with a coach service), I'd need to know your best starting station (Burgess Hill? Haywards Heath?) and the time that your niece's flight is due to land.