It's not by a lake, but ticks all the other boxes.
http://www.luxury-lodges-wales.com/
Stayed there earlier on this year, and must say it's outstanding!
Looks amazing! Did you stay as a couple? how much was it?
6 of us stayed there in one of the bigger lodges. Off the top of my head it was around the £600 mark for 4 nights.
(I think you can anyway).