These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The fat suits are quite obviously just padded body stockings. They don't look too bad when the Slags are fully clothed, but the full-body versions are totally laughable once you get past the Squick.
The animation used to make Sean's eyes pop when Sandra accidentally bites his penis off looks like it was done with one of the "goo" programs that were popular on home PCs circa 1998.
Then there's the rat during the vent scene. The CGI rivals (and possibly even fails against) Video Brinquedo.
They Just Didn't Care: By the 20-minute mark, the Slags are no longer in-character, are no longer factory workers, and are no longer in Northern England. The existence of Baz and Dave was shoehorned into the film last-minute and is irrelevant to the plot. They aren't in character, either.