Sorry I cannot help on the multi save offers bit but I paid about £70 last year from B&Q for about 80m ( 4 bed house )of 170mm rockwool to get my levels to 270mm and it was worth every penny
Part of my loft was boarded out with T&G boards and I found there was no insulation beneath em, I stuffed the whole lot with shredded rockwool and it did the trick a beaut ok its only 100mm but with the insulation provided by the floorboards perhaps it is enough and I doubted the value of putting “some height” into the loft. I have had a look at our roof when there is a frost down and the roof is white now with no thawed out spots so it is obviously working :D and the wife is not moaning about being cold 8O
We Also had the cavity walls done last year to on a free grant and the combined effect is brilliant the boiler is working alot less
My oil consumption has been halfed this year ok it is not so cold but at £700 a fill the insulation is worth every penny basically it has paid for itself already