The Brits among us can correct me, but:
-- For one thing, scripted shows are all written by one writer or perhaps a couple of writers, who have the ultimate creative control and aren't required to come up with 22 scripts a year whether or not they have 22 good ideas. If they just have four good ideas, that's what they do. If they're running dry, the show goes off for a few years until they get inspired again.
-- Especially with the taxpayer-funded BBC, the audience is believed to want a more frequently-changing schedule. (One could say that the old show biz saw of "always leave 'em wanting more" could apply.)
Anyway, one of the facets of more reality shows on U.S. prime time is, with many of them based on European formats, they are designed to play out over a fewer number of episodes than 22 a season. Of course, what we call a "season" the Brits call a "series"--"Have I Got News for You!" has twenty-some series over about 15 years because they generally do two series a year (which seem to me to usually run 13 weeks), and each one is counted separately and not considered a one-year "season."