All my flooring joists were 5.9m long and 254mm deep, but depending on position, had flange widths of 72mm, 97mm or 122mm. The flange width is as important as the joist depth, but I haven't seen the flange width mentioned, although I must admit I've only skim read this thread. I would ask whether it's possible to use a less deep joist with a wider flange. I used Scotts, https://www.scottste.co.uk/