(See: Gower and Davies 8th Edition pp114-118 and Hicks and Goo 6th edition pp83-91)
The company cannot be bound to a contract before it is formed either when the contract is made or later by adopting it – Kelner v Baxter (1866) LR 2 CP 174 but the person who purports to make the contract on behalf of the non-existent company will be personally liable on it “subject to any agreement to the contrary”– section 51 of the 2006 Act implementing art 7 of the First EEC Company Law Directive.
Any “agreement to the contrary” must be clear and explicit. Signing “on behalf of the company” or in a similar way is not enough on its own –Phonogram Ltd v Lane  QB 938 CA
Read Hicks 7 Goo 5th Edition pp 70-75 for relevant extracts from the earlier case law and the precise wording of the section and see Braymist Ltd v Wise Finance Company Ltd  EWCA Civ 127  Ch 273