Those signs do look rather homemade. I don't know if this sign
has any official use in the UK.
I don't know Southend but I think shared space can work with heavy traffic. Often it simply has to, where there are no pavements in towns. Presumed liability (or whatever the phrase is) helps here, it's often the only reason traffic takes any notice of pedestrian crossings in Poland!* But even that won't make the traffic stop for you until you're on the crossing itself - UK drivers are generally some of the best worldwide for this anyway - so it also takes a bit of nerves/balls/courage, delete as preferred. Just step out and they will stop because they have to! Obviously this only works at town speeds, but that shouldn't be a problem on this beach road.
However, if you're having to cross the road in the first place, then it's not truly shared space.
*I don't think Poland has a general liability law in the way that Germany does, but there is a specific law applying to pedestrian crossings.