It's slightly more complicated then that.
Each unit has their own specific reason. While it is true that P228, P226 do not have manual firing inhibit levers to deal with, it also means the operators have to deal with the longer and heavier DA trigger pull for the first shot.
With that specific issue, it's a matter of preference. I decided the burden of DA and the risk associated with it was less than the burden and risk of having to manipulate a manual firing inhibit levers, especially when medium pressure DA such as SIG DAK, Glock Safe Action trigger, etc. are available. Number of Europian special forces have gone with DA/SA or Glock. Having to flip lever a lot is not desirable in combat. With the amount to training SAS gets, I'm sure they can manage either way. But, thenagain, why deal with it when you don't have to? But, at the same time, number of US Special forces still uses 1911 with manual firing inhibit lever for its great single action trigger.
Also note that , in case of SAS, even though Brownig Hi-Power was a good reliable design for it's time, it's an old design. SIG has a good repulation for reliability, and I'm not sure if a Browning can match more modern engineering. Also, the trigger is heavier than the other popular SAO pistol, the 1911, and its manual firing inhibit lever is not so easy to manipulate as 1911, according to user reviews. If the SAS was to replace the Browning with another SAO 9mm, they have a very limited choice. But, there are lot more choices of quality 9mm pistols in the DA category.