That's from BFI, not Criterion. They don't distribute in the UK. Also, BFI may not have a good master of the film, and IIRC Criterion announced a new version coming soon that may be worth it to wait for:
This is just to confirm that the new BFI edition does feature the extra 25 seconds (sourced from a 35mm print as opposed to the original negative, so the picture quality changes slightly at that point) - but also that the Blu-Ray is definitely Region B.
Criterion says the missing scene doesn't appear in the interpositive or in any of the archival material they checked, including the Pasolini Foundation's prints -- which would make sense, given that it isn't in the negative either. They don't explain why the UK version ended up with the scene (it seems to me a call to the BFI would sort that out) or why it wasn't included as an extra on their release.
The BFI sourced its original DVD of Salo from an unsubtitled 35mm print that subsequently ended up in the BFI National Archive. As a result, when mastering it to HD for the Blu-ray and new SD-DVD it was relatively straightforward to telecine that print to HD and drop the scene in - the only downside is that despite extensive efforts to get the colours to match the rest of the transfer (sourced from the original negative), the scene is noticeably contrastier. However, because it's 35mm-sourced, we're emphatically not talking a Wicker Man situation - this footage is as high-resolution as everything else.
As to whether or not the footage should have been included, there's no obviously correct answer to this. True, it's not present in the original negative - but it was present in a 35mm print, presumably struck in the late 1990s, and no-one seems to know where it came from or why it was removed. (Or who removed it). And since the original BFI release featured it, albeit unwittingly (no-one noticed until someone did a comparison with the Criterion), it made sense for the new BFI release to follow suit. It certainly doesn't unbalance the film in any way.