Some buyers will find every possible excuse to push the price down, but other buyers won't. In fairness, it's sometimes a matter of bluffing as all a buyer can possibly expect is to get the house at a fair price. A relative of mine recently sold a house she had owned for nearly 30 years and it had had DIY electrical work and DIY plumbing work, but she had no trouble selling as the buyer knew that it would be difficult to find such a good house for the money involved. Certainly the place was worth no less than had it been maintained exactly as originally purchased and so was not slapped with the 'in need of modernisation' badge.
That said, if you're planning to install a new luxury suite where there obviously wasn't one before, then, having DIYed it, questions will be called into your competence and how well the work was done and then building control sign off might be desirable. Especially if you're actually planning to sell the house and want to make the new bathroom a selling point.
If, on the other hand, you currently have a tired bathroom and are simply rearranging the waste branches from the appliances to the SVP and renewing the tiling and appliances, then, in 5 years or whatever, you don't even need to make a thing about it being a new bathroom or otherwise, but it will be appreciated as a nice room. You probably won't add monetary value over the clean and functional bathroom that is there already, but a nice bathroom is always a bonus and the age probably won't even come up as the bathroom has always been in that room. And I would be highly surprised if anyone would be able to know - or care - how old the waste branches are, and if they have been changed.
That said, I do feel that, even though my theory was quite good before I did my NVQ, I would do a better job now than I would have then. Not so much because of what the course itself teaches you, as to having asked all and sundry difficult questions while I was studying.
I'm not sure having the NVQ2 paperwork itself makes much difference either way as to become an approved plumber for Water Regulations (i.e. water supply side only) you'd need NVQ2 plus Water Regulations (but level 3 is really considered the gold standard). And, even then I am not aware of any qualification that makes someone able to sign off an additional bathroom as a competent person without involving BC. The fact that the work is DIY or not is not what makes it notifiable!
As others have said, have a chat with building control and find out what they would charge. Get an electrician in for electrics and get certificates. And bear in mind that, at present, the value of a house is largely dictated by its location.
