P&O Cruises often charter Thomas Cook aircraft for their Winter Caribbean Fly/Cruises and we have been unfortunate enough to fly in them to and from Barbados(9 hrs) to Buenos Aires (18 hrs) and back from Brazil(13 1/2 hrs). If you have the opportunity to upgrade - do it ASAP. The seat pitch in Economy is dreadful and even worse the aircraft are configured 3-3-3 which is awful too. On the positive side, their crew are reasonably efficient and the seats have individual seat back screens.
Thomas Cook Premium cabin has a seat pitch of about 37" which is more bearable, and is configured 2-3-2 so a bit wider too. They are leather seats so smarter, and they have a footrest, though that does tend to get in the way sometimes.
If you can manage it, definitely upgrade - we do.