I think they're saying "believe" not in the same way as "I believe in God" - where there's no evidence. More that there's certainly some evidence to suggest the use of chemical weapons, but a) to what extent, did some get damaged, people got hurt, rebels found the bodies and claimed "Assad is using chemical weapons!". b) By which side? Both are doing horrible things, as tends to be the way in the Middle East (And any war really, but I don't think any US/UK soldiers ate Iraqi hearts), and c) Does that justify intervention when we know even if we kick out Assad Syria's just going to be a hotbed for terrorists waiting to get back at us because while we saved their asses, we didn't pray 5 times a day, or we treat women as equals, or they're bored, or something like that.
Saying "we believe" is just being politically pragmatic. The fact of the matter is that Assad could use nuclear weapons (if he somehow had them) and any intervention would only lead to more violence.