Unfortunately this reads as though you have a grey import. Unfortunately is the wrong word to be honest, as grey imports are not bad things, it is just Canon UK may not honour any manufacturer warranty, and any claim may have to be handled via the dealer (your contract is always with the dealer anyway, and as such they are legally obliged to honour it - at least a year anyway). They may choose have it repaired by a local repair shop, or send it to Canon (Japan/USA or wherever), which was the way it used be handled. In the mid noughties, the large UK retailers sought support from the large manufacturers to deal with grey imports, but due to our duties/taxes they could not offer financial support(regarding pricing at least). So they arranged it so that grey import warranties would not be covered by the UK subsidiaries. This meant at the time, some warranties were very limited, in both what they covered and time. (this is what we were advised at the time when we were getting hammered with hardware sale competition from foreign e-tailers)
I would not be concerned if it is a grey import (2 year warranty on a Canon is normally a giveaway), as it is still a quality product, and built to the same standards, in fact should be identical to a UK bought product.
Your question to them in my mind was not direct enough and therefore their answer could be interpreted as correct. It is a camera made for the UK market, as it is identical to a UK camera. I do not think there are any variations worldwide on dual and single digit models (1D, 5D 6d 50D 80D etc). Likewise it may have been opened for display purposes, or demonstration. It could still be classed as brand new, just not sealed. In my past life we have opened boxes to check contents or compare etc. Test the camera by searching for
ACTUATION COUNT, which should tell you its life.
Additionally, check the condition of the sensor, by placing it on a table, setting the lens to manual focus, focus to infinity, set to 'A' and set the aperture to its minimum (largest number f29?). Switch on the timer release for 2 or 10 seconds, and set the exposure compensation for +1/2 or +1. Put a piece of white paper or card about 8-10" from the lens and take a shot. The resulting image should show any marks on the sensor and possibly on the lens itself depending on where you have zoomed to (which you can experiment with).
I too like to keep the boxes, still have them for my bought new lenses and cameras, and the instruction pack is still sealed for the camera. Canon instructions packs used to come in a sealed pack with the software disc(s).
I would do the tests above, and if they check out, either content myself with saving £400, or request a sealed instruction pack. If they didnt check out, then get back in contact with them, and I believe you can use consumer contracts regulations to request a return and refund - check out their terms of sale for the full details, but I believe you have a minimum of 14 days to arrange this. The Which guide is
HERE.