If you are over 50 then Saga is the line for you.
P&O Artemis, adults only, slightly over your requirement at 1188 passengers, 45,000ton.
Try any of the luxury brands like Seabourne, Oceania, SilverSeas or Azamara - small ships that do not encourage childre.
Fred Olsen.Black Watch or Boudicca, classic ships with about 850 passengers. Sail from a wide variety of British ports.
It will have children, but not a lot of them, particularly if you don't sail in holiday time, and those that I've seen, were seen and rarely heard.
I agree with most of the above. If you can travel late in August the kids will be back at school so there is less change of being overwhelmed by kids. If you see an advert that says 'free child places' or family cabins available, give it a miss. Thomson Emerald is small and although it is not strictly child free, it is a ship that does not cater too well for them and I have never seen any aboard.
Fred olsen ships carry children but not a lo.