We need to get from Heathrow Terminal 3 to Watford Junction with luggage (carryon). It appears the most efficient way would be either the London bus or National Express bus. The National Express website, however, says it's operated by a 3rd party, Green Line (Excel). It's about an hour ride and £20 for two people. I found a Greenline (one word) website, but couldn't find any information on this supposed route. But then the Transport for London website shows a similar bus route #724 leaving the Heathrow bus terminal for about the same price. But I can't find the #724 bus elsewhere on the TFL website. So I'm totally confused if this is the same bus or different one. And would they have room for a carryon suitcase (or I can hold in my lap, I guess). Or is there some other option such as a shuttle or Uber?
Would much appreciate helpful advice.