I've got a couple of SH800's and have tested them in some pretty ropey conditions. All positive so far.
As mentioned above, buy a 13L airlock dry bag and it fits perfectly.
In terms of warmth rating.....it's so difficult to put a realistic figure on bags. So much can affect how warm you feel when you get in (humidity, tiredness, hunger etc), so even getting in the same bag feels different every time.
But, as for the SH800's I can safely say that you'll survive pretty much any UK temps, provided you and the bag are dry. I recently bailed on a climb and wild camped. It was around -5 and I was very wet/cold/hungry/tired. I managed keep sufficiently warm with a pair of long johns and a fleece.
As for build quality......it's actually very good for £140. Compare it to the likes of PHD and you'll notice the difference, but you'll also pay triple the price.
One tip.....buy a silk/cotton liner. It's nice to be able to jump in your bag with dirty/sweaty kit on and not have to worry. Once a down bag starts to smell....well let's just say it doesn't go away very easily!