Ah, so it is just the way those shows worked out then. I wasn't sure if maybe it was trend in British TV for actors not to last more than two or three seasons.
No, it isn't specific to those shows; it's pretty rare for anyone to last more than three or four years in a single series on British TV. Mind you, it's pretty rare for a series to run more than three or four years in the UK - though it's far from unknown, and a series which gets past that point might well run for 10 or 20 years.
(A few examples: Dixon of Dock Green ran from 1955 to 1976; Z Cars ran from 1962 to 1978, with three spin-off running alongside it during that time; Casualty started in 1986 and is still running; its spin-off Holby City started in 1998 and is also on-going; Midsommer Murders started in 1998 and is commissioned through to 2011; and Heartbeat started in 1993. Plus there's the three 'primetime soaps': Coronation Street, which started in 1961, Emmerdale, dating from 1971, and EastEnders, which began in 1985).
But these all tend to have the same degree of cast turnover, with occasional exceptions when one actor decides to stick with the show (Derek thompson has been in Casualty since 1986; Jack Warner was in the entire run of Dixon; Stratford Johns and Frank Windsor left Z Cars after three years, but only so their characters could go off into the spin-off series).
Christopher Bennett put his finger on part of why it happens in another post, and it is probably down to 'how it came about'. I suspect it's that, with US filmed TV series usually made by the TV divisions of Hollywood movie studios, the old 'Hollywood star contract' of the 1940s mutated into the standard seven year option which actors sign for US TV series.
Whereas in Britain, with the British film industry dying as TV took off, actors got signed the same way they would be for a stage tour; you sign for the run, with an option for any extension, and then it's your choice whether to do more beyond that. Hence one season, then maybe a second, and then you can stay or go.
Sorry: a bit of a long post given this is a bit O/T.