I would have a word with the local fire brigade, something doesn't sound right.
The installer should leave the installation compliant, that includes sealing the door to frame correctly, here should be no gaps and it should be fire stopped (this is a legal requirement). I would request a copy of the installation certificate (FIRAS, TRADA or the contactors competency to instal fire doors)
The price, for a fully installed certified fire door is in the right ballpark, it’s the certification of the installation that costs
The decoration is debatable, the cost between a fire door installer and a decorator can be considerable do on a large job it is probably best to employ separate trades.