Can't speak to the UK market, but 512GB SSDs are commonly in the US$210 range these days. I bought a Crucial mx100 512GB SSD in late June for around that much so it's held for that long at least
The prices in the UK (on Hotukdeals.com) for Samsung and Crucial SSDs have been falling for a while. Not sure when they'll hit the bottom though. Maybe just before a NAND factory fire in Korea
I'm tempted by a 512gb SSD for my Mac Mini, but holding to see if a new Mini gets released.
SSD prices have really fallen over the last two months or so in the US, so maybe the UK is just a bit behind. It may correct a little bit, but this is definitely not just a temporary sale by any particular retailer or manufacturer.
About a month ago I bought the 500GB Samsung EVO 840 you mentioned for $229 US from Newegg. Pretty awesome.
Though prices of SSDs are slowly dropping in general, in this case it's more likely that they are clearing out the 840 model for the incoming 850s. The 850s will be closer to 70 cents per gig.