It is somewhat similar to unarmed combat training. Most soldiers will not spent most of their time in hand to hand combat, and thus the training is not necessarily the best available. It is just the best available for someone not expected to spend most of their time in H2H combat. Thus, the perfect style for the non-H2H combatant. Hence the likes of MCMAP for the US Marines, and the basic levels of Krav Maga for normal Israeli infantry. Special operations forces definitely get better H2H training (e.g. DEVGRU and CAG do get instructors who teach some rather interesting things ranging from Apache knife fighting, Sayoc Kali, and some rather eclectic stuff like P.F.S and S.P.E.A.R; while the Israelis teach harder types of Krav Maga and KAPAP) ...but still ...these guys will spend most of their time practicing on their main weapons.
For the average soldier (read: not part of special operations) the time spent working on their side arm is less than that done by the average police officer, and the Glock is absolutely perfect for the average soldier. A member of DEVGRU or Sayeret can make a case for a different handgun, but the average soldier ...not really (apart from personal taste). The average soldier is a 'non-shooter' when it comes to handguns as compared to i) the amount of time spent on the handgun vs the primary weapon, and ii) compared to even civilian shooters who have decided to put serious time into handgun practice. Consequently, if the Glock is the perfect gun for a non-shooter, the British chose wisely.
Thats all nice but why does that make a Browning better then a Glock.
What I see is a lot of reasons to chose a Glock over a Browning.
With the growing green on blue attacks, soldiers will need more training with sidearms.
It is true that you use a pistol in the same circumstances that you'd use a knife but there is no excuse for carrying flimsy item just because it isn't used often or it's cheaper to buy.
As a Vietnam Vet, I'm sick of risking our lives on "least bidder" weapons. If the Brits have to go back to the Falklands, they'd better keep a few Brownings available, just in case.