I started a search like this a while back and had similar problems, in the end we have just bought a 50" TV, bigger than we were looking for but it is fantastic!
We went for this one:
http://www.panasonic.com/uk/consumer...-50dx700b.html
There is a 40" version so should fit.
We don't particularly need a 4K HDR TV, the only source of that quality I have is Amazon which streams straight to the TV. Other than that it's Sky HD and a blu-ray player for which I have 3 blu-ray discs!
The main reason I went for this over a Full HD or whatever else is that it gives some degree of future proofing should we decide to get more 4K content etc.
The user interface on it is very good and the online content all seems to work very well.