Have to say I've only ever seen external frost stats on older boilers where it was fitted in undercroft or outhouse, garage etc..
Our old 1994 boiler didn't have one.
Then like a lot of things its built in but they still recommend external. Like, our boiler has bypass but still recommends you
fit an external one. The original plumbers didn't but I have... Same with pressure vessel, it has one built in but for a larger
house it recommends an bigger external one, which again the plumbers didn't but I have! The plumbers were working for the
previous householder whilst the house was up for sale so no doubt were told not to spend a penny more than needed. That
said its not a cheap, nor bottom of the range boiler.