Yes, as noted, Air SouthWest is pretty small. Depends on time-of-year but fog in Cornwall can be a problem outside of the summer months.

My mother has used them - one nightmare journey involved a delayed take-off from LGW hoping fog would clear Newquay. Well it didn't and they were diverted to Plymouth where they had to wait for a coach.

Now she generally sticks to the train unless there is a real price advantage going by plane. But Penzance (where the train terminates) is better for her than Newquay (where the airport is) anyway.

As for Easyjet, I've found them OK but for low cost carriers I've found Ryanair more reliable. Only problem with Ryanair out of London is Stansted which I hate.