Depending on what you mean by "English", the longest station name in English appears to be "Rhoose Cardiff International Airport"
The longest station name in England is "Ashford International (Eurostar)"
Then "Prestwick International Airport", "London St Pancras International", "St James Street (Walthamstow)" and "Southampton Airport Parkway"
Station names taken from here:
http://www.nationalrail.co.uk/stations/codes/ (ignoring the Tube stations, such as "High Street Kensington Underground")
Edit: I've just spotted that NR have abbreviated "Manchester Utd Football Gd". Assuming the station name is unabbreviated, "Manchester United Football Ground" is longer than Ashford International (Eurostar), but shorter than Rhoose Cardiff International Airport