pls advise- best insulation, sub-floor and underlayment for tiled kitchen floor?

[diywork]

My floor joists are good but I need to replace whole subfloor and tile kitchen floor. I am working on a budget but I want to install functional and durable solution according to British standard for both subfloor and underlayment. Ppl with experience, could you please advise what insulation to use for floor? should I use plywood, cement board and decoupling membrane? Or cement board direct on joists with decoupling membrane? Also there are these tile backer insulation boards, would that be the right move? Was thinking to avoid membranes such as ditra as more pricey options...

 

[J K Tiling Cornwall]

Hi, firstly cement board cannot be fitted direct to the joists so the first option would be a structural flooring grade chipboard or plywood * ideally water resistant. If T&G use glue and try to use extra screws and put in additional noggins if needed to support joints in the boards. If all good and solid with no deflection I would over board with a foam board (bedded on adhesive and screwed down using washers). Joints can be fibreglass taped. Then good to go. But everyone has their own preferred method.