Blown window replacement upvc double glazing

Hi all,
I've been thinking about replacing the glass in one or two blown windows and naturally turned to youtube and a few other places. It seems quite an easy job, just pop off the beading take the glass out and replace it with new glass. My question is; is this allowed under building regulations? Where would I source the glass? Is it just a case of finding a made to measure supplier online and getting it measured up? The inside scoop would be most appreciated as I would like to have a go at DIYing a replacement myself. Many thanks

As you are replacing not renewing building regs are unaffected.

Source the glass from a double glazed unit manufacturer who supply trade and to public, they'll need sizes I millimeters not inches, width always first then height, then overall thickness, then colour of spacer bar, they may ask if the inner pane is a low e reflective glass but without a tester you won't know, if it's feasible could you take the units out and take to the glass shop, they could measure and test do you'll know they'll fit and look right

Thanks so much. Questions answered