How do I bathroom / kitchen / extension? etc.

Try Munster Joinery, they were much cheaper than Rational for us:
https://www.munsterjoinery.co.uk/

We got a similar amount of glass: 10 windows, 3 doors and a large french double door (all Passivhaus triple glazed units) for £7,250 fitted.

Submitted to suppliers for pricing on velfac / rational / flush aluminum. is a lorra lorra glass. guesses for supply and fit please - winner gets a £5 boots gift card

So what's the damage then?