Not as such AFAIK (unless a HIP requires one).
If you disclosed that recent work has been done on the place then they
may take more interest. Having said that the standard sellers forms
allow for the possibility that paperwork is "lost".
Why would a test fail if you have done the work correctly? (or am I
missing your point)
Yes, why not.
The colours don't date work as pre or post part pee anyway since new
colours were allowable before, and old colours allowable after.
Why should it fail? If you do the job correctly you'll likely get a
better result than so-called 'professionals', simply because you know
that you are doing the job for your own benefit and safety rather than
as a money-making venture, so you will use decent quality
wire/cable/accessories/miscellaneous 'bits and pieces', and follow the
If someone has to test it, it makes no difference at all who carried
out the work. As long as it's a 'satisfactory' installation the
certificate will (should!) be forthcoming.
Colours of the wires don't affect safety.
Regarding selling a house as you mention, forget that you intend to
sell, treat the house as your own and develop it accordingly, then you
have nothing to hide from a potential purchaser.