I so wanted to say HF too, but didn't know if we could. Nail on head. They are great and I've done several trips with them. Special interest ones are good - natural history and walking as an example - and my first trip to America, starting in Boston, then travelling in New England, was wonderful.
You sit at large tables , any spot that is free, and the ethos of HF is friendliness. Prices appear high, but as has been said, you don't need to spend more except for when there is a tea stop. Lots of people are solos.
Nature trek are also good, with a bias on natural history, and Just You is another company. My husband is not a natural joiner and knee replacements stopped him doing significant walking. I have now reached the dodgy knee point too so wouldn't do another walking holiday. HF also offer singing, yoga, gardens and much more.
Do let us all know how you get on.