Have consided a cheaper insulation than celotex...mineral wool for example, then osb if you want lower cost, timber frame houses use 9 mm I believe. You could use insulated plaster board, or roofing boards too
This site gives prices to work from
https://www.insulationsuperstore.co.uk/browse/ceiling-roofing/timber-products.html
I think it's about £12 per roll
Ughh! I've just looked. I'm well out of touch. It's about double that now.
I remember getting a load of this on some govt scheme at £1.20 per roll, and then wincing when it went up to a tenner when the scheme ended. It's more like £25 a roll now!
Some places asking £40 odd quid?