Explain to me why that is not something people can vote on?Because there's this thing known as "freedom of religion" which is recognized both in UK and European law (to which the UK is still bound) as fundamental and inviolable.
Meaning: no you can not "vote" on the question of whether a certain group is allowed to practice their religion (yes, in your neighborhood) -- and advertise this fact. The size of the building, or the width of the signage can be regulated perhaps... but the very fact of that house of worship being built -- sorry, no.
What if I don't want Saudi funded mosques in particular, is there any nuance allowed?
The "No More Mosques!" campaign clearly is not limited to foreign-funded mosques (or those funded by nasty regimes); it is explicitly aimed against building mosques in general.
That's why there's basically no way to look at it as fundamentally anti-Muslim.
Should we lock up Richard Dawkins for being anti-Catholic?
No, and no one is saying anyone should be "locked up" for carrying those banners, or running those ads.