Are you saying UK and US speak a different language? No one doubts it’s a different dialect, but we are talking about languages.

that is a political distinction

Mutual intelligibility is not a political distinction.

Well yes, the dialects are very distinct linguistically, but what is often referred to as Flemish is the Dutch "tussentaal" aka "verkavelingsvlaams"[1]. That's not really a language per se, it's a regiolect of the official Dutch language, itself a Dutch variant of the Brabant dialects. The Flemish Dutch is usually used a lingua franca because the official Dutch otherwise sounds too formal (and native dialect speakers are foreign language speakers of Dutch). If I was to nominate a regional language for recognition it would more be the regional dialects like Brugs, Gents, Antwerps, Brussels Vloms, Hasselts, etc.

What I find interesting is that the differences in Flemish dialects make them much more distinct than what would normally call dialects. There are significant grammatical difference beyond the usual vocabulary differences. For instance, coastal Flemish conjugates yes and no[1], Limburgisch is a tonal language.

[1] https://nl.wikipedia.org/wiki/Tussentaal