John Lewis, mainly because they had a special buy on and had only 3 choices of spring firmness, rather than the 8(?) that Dreams etc. have. For me personally too much choice can be a bad thing and life’s too short to spend ages looking at all the options. JL helped cut to the chase. The one we bought was JL branded but is made by Harrison Spinks in Yorkshire.
Dreams don’t have every mattress in every spring firmness in the shop. In JL we could lie on the mattress we were going to buy. However Dreams can counter this with 40 night swap thing I suppose
EDIT: Also remember the bed base type affects the firmness too