Heathrow, Gatwick & Manchester Airport operate different terminals dependant on your flight, you will need to specify this when you book.

Each provides a website which can be used to check which terminal you need.
Heathrow https://www.heathrow.com/airport-guide/which-terminal.
Gatwick https://www.gatwickairport.com/flights/which-terminal/
Manchester https://www.manchesterairport.co.uk/flight-information/which-terminal/

If you need further assistance please feel free to contact our customer services team who will be able to assist, this is best done via our Online Chat or through our Contact Form.