I phoned Howden doors, and asked them for reason they don't allow any shaving from the top, the main reason they gave me was it would tamper with the label, and so the door would be no longer FD30 certfied due to removal of the label, so technically it does not effect its performance if one was to shave 4 mm off the top. I am taking my risk here against council officials wanting to see the label, in any case i managed to remove the label carefully with steamer and I will now be able to stick it back on.
I also asked what is the door composed off and I was told they use chemically treated chipboard with fire retardant qualities to provide FD30 ratings.
and whether I could drill a 175mm deep hole 16mm diameter mid way for a concealed closure, and yes I could do that.
Another important question i asked them and they were not able to answer, which has been intriguing me, let us see what your opinion on this is:
If you look at any door, you have a side with gaps all around, and you have the opposite side with door stop strip.
Now let us say the fire breaks where the door strip is as this may be the case with my kitchen, where the door stop strip is nailed into. How will this effect the heat reaching the intumescent strip since the heat would take a while to reach it due to stopper bar. This means the seals may not activate for a while and smoke may pour out from the gaps for much longer.
On the other hand, if the gaps were towards the kitchen and door opens into the kitchen, then the heat would penetrate the gaps and reach the intumescemt strip much earlier, and activate much more quicker and stop smoke seepage.
Though in my case I could have hinged the door either way, but since no one gave me any specific information, I left the door hinge at the same side where the old door was hinged i.e. opening into a lobby and not kitchen.
Another serious suggestion I came up with, which could possibly remove the problems associated with uneven door gaps and and gaps around hinges is to provide tight smoke seal when the fire first starts initially, is to use a second line of defense by rebating a fur lined instumescent strips in the door stop, since these are now required to be bigger in size, and it could easily be carried out and should provide air tight seal, and it would also act quicker as the heat would reach it quicker if it was at the side of the fire, as well as rebating the one required in the frame, or door edge.