No, the Hilton is not «considerably farther away” than the others. But it’s not in the part of the city closest to the airport, and with a 8.30 am flight that is something I personally would keep in mind. (15 minutes extra matter a lot to me in the morning ).

I understand that you just want a nice place to spend the night. In that case the Hilton is a good choice. One of their restaurants (ground floor) has an outdoor terrace next to the river. Afterwards have a few drinks in the bar on the top floor and enjoy the view.
To win some precious time on arrival you might consider the hotel transport. It’s of course more expensive than a taxi (2200 Bath a few months ago), but I find it worth it. In the morning just take a taxi; you will not get to the airport faster if you take the hotel limousine.