If you care to go back and read my other posts in this thread (and I certainly wouldn't blame you if you don't) I have always acknowledged that it could have been someone other than BG--but only if that person matched BG's profile to the extent that the evidence would fit. The Met investigated thousands of people, including underworld figures, assassins, foreign nationals, and everyone else you could think of. Only BG fit their evidence and didn't have an alibi placing him somewhere else.
I don't doubt there is someone, somewhere, who would also fit the evidence. But the chance of there being two such people on Gowan Avenue on the day of Jill's murder seems vanishingly small. BG was seen on Gowan Avenue earlier in the morning, and he was seen nearby at the help centre shortly afterwards. We know with certainty that he was in the area, something which can't be proven for anyone else.
It's possible BG would have panicked, but his actions still don't make sense. As
@WestLondoner noted, letting the police examine his gun could have proven his innocence. Instead the weapon disappeared, and he has done nothing but lie about it ever since.
And I'd like to clarify, once again, that despite BG listing the gun under the heading "Replicas," that model of gun wasn't a replica. It was a blank-firing starter pistol. You probably already know this, but blank cartridges are still real ammunition. There's nothing fake or replica about them. They have a shell casing and gunpowder. The difference is that instead of firing a lead bullet, they fire a "bullet" made from compressed wadding which disintegrates shortly after leaving the barrel and turns into harmless fluff. However, even that wadding "bullet" can be deadly if it's fired at point-blank range; the actor Jon-Erik Hexum (
Wiki Link) died after he played Russian roulette with blanks, not realising it was dangerous.
To fire real bullets from the starter pistol would, as I understand it, have required some relatively easy modifications to the gun and probably some modification of the ammunition itself--exactly as was found on the hand-crimped cartridge cases at the scene.
None of it proves BG is guilty beyond *all* doubt. But IMO it does prove his guilt beyond *reasonable* doubt: Because if he didn't do it, there must have been two BG's on Gowan Avenue that morning.