The reason it seems more believable OYBBK is because it is.
The Blair dossier was based on assessments, based in information from sources whose veracity was good enough - but at the end of the day claims such as that could never be verified. And Blair, when asked if it has been 'sexed up' (ie too much weight placed on the parts of the info that supported what he wanted to do in the first place) ignored the actual question and instead started to deny the government had fabricated intelligence. Which tends to show it was all about spin and getting your way.
In this case, the government is saying 'We have this material in our labs and we have analysed it, and we can see actual scientific evidence of what it is. We (and everyone else) are unaware of anyone other than the Russians who have ever made this stuff. We have invited in OPCW to verify (and they either arrived yesterday or are arriving today)'.
So, if Porton Down have made a mistake in the analysis, it will be discovered. If, as I believe, scientists that specialised, looking at something that is international political dynamite, will double and triple and quadruple check everything and it is therefore vanishingly unlikely that a mistake has been made. So this isn't assessment and spin. It's fact. This is the substance.
No-one else has ever been known to make it. That doesn't of course mean no-one else ever could. But there really doesn't seem to be any suggestion whatsoever that anyone else has, or (aside from US or China) actually could.