Some areas will fit loft insulation for free...for anyone...
Might be worth a call to the Energy Saving trust to ask for advice...
I could have had Cavity Wall done for free in house I bought (I'm just a home owner - not old or on benefits etc) ...but it had already been done (surveyor missed it for EPC)
Only problem - your loft will have to be completely empty - you can't do it in areas and move stuff around - they will just put it over whatever is there - won't clean up any dirt etc - (apparently have been know to put insulation over a box of Christmas decorations - the last box, next to the hatch someone forgot to move for their elderly mother!) - and it will all over loft roll to 270mm - so you can't have any storage in there ...don't know what the score would be if you had already fitted loft legs/raised the joists????
If you can get it for free - you could get them to do it and then you go and roll it up in areas and use it how you want to - but that will be messier and bulkier than unrolling new packets ...will restrict your moving space...and you won't be able to see the joists... make it a harder job.
I know - I have been doing this - the house I bought had had this done (original loose fill and two types of roll - guess topped up as depth requirement had increased) - I'm trying to make just central boarded area...sometimes wish I could just get rid of it and start again - but actually getting rid of it would be a nightmare too.
(I'm also finding bits of old kitchen unit must have used as flooring at some point but now randomly hidden between the layers - put foot on where joist should be and it was the corner of one, it tipped up and I nearly went over

)
The stuff in the sheds is subsidised and they often have special offers (3 for 2) - so might be worth watching out for them -
Wickes used to be cheapest when I did attic in present house - compare the sizes...roll length and width. I found the B&Q roll was slightly smaller than the similar
Wickes one -
Found this in board underlay too- seemed same spec etc - B&Q cheaper per packet - then realised it covered less area (contained 19 rather than 20 boards)!