Yes, I think that the NBL site is not yet complete as I could not find a number of events I know should be there. Still, from little acorns....and in the meantime there is FreeBMD, which is a project run by volunteers and an amazing achievement. I myself find it very flexible, clear and easy to use, though it is great when there is also a local register office on-line index to cross check.
Don't forget that the NBL site will not cover, for example, Newcastle on Tyne, or North Tyneside, both of which have their own websites. People from quite far north would sometimes get married in Newcastle - perhaps the bride was in service there, perhaps there were other reasons.
No, unfortunately you cannot download 'uncertified' or any other kind of BMD certificate for England or Wales. There is also less info on the certs than is the case for Scotland.
However, on the plus side, the GRO indexes are free to view, and civil registration started a lot earlier (mid 1837 compared to 1855). Scotlandspeople charges for viewing index entries and with such a limited range of names in Scotland it can get expensive checking possibles - so many do not do this. I have not found my Scottish research cheap or easy - on the contrary.
So perhaps there are pros and cons for both systems

Matilda