I think the main issue here is that you will have to deal with the hassle of reregistration of the vehicle as a 7 seater. That is if you want to use that 3rd row of seats for people, not only dogs or cats

I don't know much about french or british vehicle registration but I think it must be pretty much the same as in my country. The vehicle should be first (re)certified ("homologation des vehicules" en francais) and this means that all the modifications need to be made through the authorized LR dealer network (you need some papers from them stating that).
As for the parts needed, additional or different between a 5 and 7 seater, they are a lot. Just have a look in the parts catalog in the trim section only.