The fat content of French creams are nowhere near as high as double cream so there is no substitute with the right taste, fixe chantilly is rubbish IMHO, you are already lacking the base cream flavour of double cream, the fixe either adds a strong cardboard taste or very sweet cardboard.
Crème fraiche, acidic but i love the tasts and use it for all sauces, I never use flour now.
Fromage blanc, to me less acidic taste than yoghurt, more bland, low fat but great as a cream substitute, I get through a couple of kg per week and make my own from UHT whole milk.
Where a double cream is needed for whipping then use Lidl crème mascarapone, the cheapest, I like the taste and its even hgher fat content you may want to dose it with single cream.
Flour I never use, I still have the same bag and also the bag of sugar that came with me in 2004!
Maizena I use in place of cornflour for thickening, its the same but different somehow, I prefer it, its also great as a laundry starch.
Editted, faiselle is a step closer to cottage cheese from fromage blanc and comes with the whey protein that would be drained from cottage cheese and sold as protein powder to body builders, dont throw it away, drink it!!!