8 stones and looking for a soft bed

Hi guys, I am 31 years old, weigh just under 8 stones and have a very petite frame at 5ft 1 inches tall.
I am looking to replace a mattress onto a metal bed frame which has wooden slats. The slats are around 10 cm apart.
I would like a soft mattress, something with a bit of bounce, or that sinking type feeling when you lie on it. I understand this can be achieved with a topper. But would still like something soft-ish underneath that. Pocket sprung with no memory foam at all, I find it too tough and wake up feeling as though I have slept on a bare and hard floor.
Before going to the shops original budget was £400, when I got to the shops I was finding the ones I liked where between £600-£800. I would be comfortable with spending £500, £600 if it was the "perfect" one but you can only tell that after having slept on it for a few weeks I guess.
Favourite was a Dreams mattress, from the flaxby range, natures finest elite 3000 in the soft tension. It had the marshmallowy type feeling, very soft. Fillings were cotton and white fibre, not premium enough to command the £800 price tag in my opinion. http://www.dreams.co.uk/natures-elite-mattress
Next was John Lewis natural collection cotton 3000 which starts in the medium tension. Not as soft but still very comfortable. Fillings are cotton and wool and hemp. I found hemp tough on some mattresses where it was in the top layer, but in this mattress it was lower down, directly above the springs so more comfortable at contact point. Price of this is £600 http://m.johnlewis.com/mt/www.johnlewis.com/john-lewis-natural-collection-cotton-3000-mattress-double/p557311#page_loaded
My cousin has bought a mattress of you, the Origins Pocket 1500 which I have tried. It is very hard for me, and I found turning and tossing on the bed was hard work. I think he must have the firm tension. From your blogs I believe this range is the softest one that you do, in particular the Origins Comfort. But for my weight range it says it would feel medium. Would you recommend a topper to make it softer on top and if so which one? Or do you have something which would feel softer to begin with? Do you need any information regarding sleeping style to be able to recommend?
Thanks in advance!