Given it seems to be a continuation of prior fencing off, the issue seems to have been a long time coming: when the properties got sold, when the other neighbours put up a fence. With the layout of houses and proximity to the fence, I fear by now it's a lost battle

Luckily there is a much larger, nicer beach on the other side, about a 10 minute walk, where dogs are allowed year-round.

But really what are the owners going to do? Police aren't going to care about a trespass. A CCTV with a speaker? Super scary ! They're just banking on people heeding the signs. Very England. Why don't the 650 people get together and meet on the beach?

I thought that all lands between high and low tide belongs to the monarch. Is that not applicable here?

No idea about the law in this area. My first sentence wasn't meant as support of any fencing off.