They'd be a lot more readable if the ludicrous decimal place was removed, or agreements reached to round the price up universally for exterior signage purposes even if they wish to actually charge 'only' 130.9 at the pump itself, is anyone normal actually conned into thinking 130.9 isn't really 131?

The need them on both, as the high up references are likely for other drivers and especially the staff in the kiosk/shop so they can easily identify the pump by looking through the window or on their CCTV screens.

Filling stations should also display ALL their fuel prices on the large sign at the boundary of the site, and not just the standard petrol and diesel. I've seen (not that often but a few times) people baulk at the price of the superfuel (at a branded filling station), especially superunleaded, and then block the pump because they can't get out because they are blocked on each side by other vehicles.

That could/would result in unreadable signage - with potential prices for unleaded, premium unleaded, diesel, premium diesel, adblue and LPG

Nah - I don't believe that - most of the signs are full of nothing. There's more than enough room for them, give signboards of the same width and height are perfectly readable for retail parks showing all the shop names. All they'd have to do is make the wording clear and readable in all weather conditions, as the best filling station signs are now.

I think they don't put up the superfuel prices in particular because they believe people buying it won't care about the price they're paying - true for some, but not all.