i also use GHW mostly nuts but do have some hex head screws in stock for the odd occasion that they are needed. You may have a job finding 2.5mm dia steel for studs but I've made 20+ engines using 3/32" steel whick takes the M2.5 thread Ok.
If you don't want to but from mainland Europe then Polly Eng keep some of GHW's range.
Not sure what the BSI specifies but generally the GHW ones are one size smaller so M4 is 5.5mm hex etc. They are also fully machined with single chamfer so look a lot better than mass produced standard metric ones.
This shows M4 & M3 standard in the middle, GHW outside

Edited By JasonB on 26/11/2022 13:11:23