I think it would take more than GBP5mil to build a DH91, Felixstowe or restore a Wellington. Personally, from my very limited replica-building experience!
Depends on how much work could be carried out by volunteers etc. If you were to hand the job over to a professional restoration company, paying them at least £60 + VAT per hour, and whatever they add on the price of any original parts they obtain on your behalf, then yes it may well come to well over 5 million.
Bob T.