In the old days, they just sent an inspector around to look at what you had done.
I found its a bit of a lottery getting installation work of BG. You can get some bad houses and have a lot of mucking about agreeing additions.
Things like, taking up a floor and finding only 2" x 2" joisting which you are expected to notch out for 22mm pipes. It then takes ages to get customer and BG inspectors agreement for the extra work involved to make a new route and then its usually against a set menu of prices.
Also chasing around finding an inspector to sign a chitty to get any extra fittings and pipe required.
We found it not worth it, unless you can see the job yourself first and agree routes and extras with all concerned.
Don't go by what appear good prices, its pointless thinking £5 a metre is good money, then getting to the job, find the pipe you are expected to fit is under laminate floors that you are also expected to lift and reinstate all for the £5.
Nice guys, probably be a good talk, but they are also business men.