having sold comp doors for a bit last year, i will try and avoid slagging the lying ***** off too much.
comp doors are better than upvc, but they are not perfect. because they are a composite there is always the possibility of differential expansion leading to the skin delaminating. This is avoidable, if your door is south facing stick to a light colour. Check the furniture is suited, some doors are supplied with unsuited furniture it does save a few pennies but you get slightly different colours for handles and letterplate, which IMO can look a little odd.
Be aware that your guarantee will be with the installer and they have a tendency to go bust with alarming regularity so if anything goes wrong you may find yourself out of luck.
I could also say the mark up on doors is frightening.
As for the police, the police had major problems gaining access to one property with a comp door. Installation matters!!!! i can not stress that enough, there are alot of cowboys, and a badly installed door is likely to cause problems.
You shouldn’t need to worry about U values as your dealing with a refurb job, but if you want to maximise insulation use as little glass as you can get away with.