Thanks for asking this question. I'm going to be in the same market in the new year when I'm guessing prices might drop. The focus on the Forum is generally about big screens but 40/42 will be an upgrade for me and will suit my type of viewing and room size. I'm also not fussed about 3D. I'd like the option of Netflix and Amazon as well.
The Panasonic comes with the new Freeview Play facility which is a great idea, but probably needs a few updates to get right. The 40" Panasonic appears to be a Currys/PC World exclusive at the moment which doesn't give much scope for discounts. JL sell a CX400 which appears to be cheaper and the same spec. (There is also a view that Panasonic might not make this TV but have farmed it out to Vestel, but this is true of other manufacturers as well. So many TVs are 'kit' built it is hard to know who makes what and where. Labels on the back frequently say 'Assembled in...' which invariably means that preassembled parts have come from elsewhere)
If you found them all at £500 I'd like to know where. Of course there will be a number of people who will say that 4k is pointless at a screen this size. They said the same thing when 1080 HD screens came on the market. Bear in mind that Sony studio monitors are 4K and less than 40", so there must be a point. The deal breaker for me will be the ability to give a reasonable picture with SD material.
Add Hisense 40K321 LED 4K UHD Smart TV, 40" at £449 (JL) to your list.