Insistence on (EDIT: having used) using type=number.

Given the choice of bad UI vs. (accidentally) modified data, I'd pick bad UI any day.

Insistence on using number makes me suspect they require sanitation to be done client-side.

EDIT: Normally, I wouldn't assume a website run by a government wouldn't be doing this, but wasn't there some scandal recently in the US about the Dems and some web-app, and that it was woefully amateuristic? Suffice to say, that has kinda changed my perception about governments and IT. Then again, "gov.uk" has a tech blog, so I'm probably misplacing my skepticism.

EDIT2: Updated tense.