I bet the SAS will acquire some Glock 18s as well, and having a 33rnd mag for the 17 in some situations is an advantage.
The SAS replaced their P35s with SIGs back in the early/mid 90's. The rank and file actually preferred the CZ75, but the MOD pushed for a weapon that had been built by a cold war ally, as the future prospects and allegiances of Czechoslovakia/Czech Republic were uncertain at the time.