Although the OU do not do a B Eng degree they do a BSc, for which you can pick from a wide range of engineering subjects. The Engineering Societies do (or at least did 5 years ago) recognise such degrees provided they are made up of an acceptable mix of modules at appropriate levels.
Essentially you have to do an honours degree made up of a mix of modules picked from a list provided by the society you wish to join (assuming of course that you wish to join any of them).
If you write to one of the socities they will send you a list of what is required. The RAES used to send out a combined list for a wide range of societes.
It is of course possible that all of this has been "improved" over the past few years.