Well if purchasing from a dealer, and they are certified Cessna parts, they will come with the certified EASA STC.
Now, this is where a can of worms can potentially open up. There are strict standards on fire certification for carpets, and upholstery, and you really have to enquire if parts being purchased, conform.
It can be an expensive mistake, if it goes wrong.
I am sure there are shops in the UK which would be glad to quote, try Eastern at Sturgate, or google aircraft fit out shops UK. You might be surprised at the cost effectiveness of getting someone to do it for you, and you could pick what bits you could do, with their input and or advice.