I am currently working towards the B.Eng. and M.Eng. with the OU.
Just to clarify, the M.Eng. from the OU is recognised by several institutions as fulfilling the educational requirement for CEng registration. (No assessment will be made of the qualification it is just accepted).
I agree that the OU lacks aeronatical specific qualifications. However, the credits you achieve for modules from other providers should be recognised and transferrable to the OU degree. (Check with both if you intend to do this as transferred credit must be relevant, at the correct level and of enough length).
Therefore you could complete an aeronautical specific component with a different provider, (perhaps full time if you could take say 6 months off and there was a suitable course), and then have the credits from this incorporated into your degree profile from the OU.
You would then be able to benefit from the flexible delivery of the OU for the bulk of your degree but make it more suitable for your needs by linking training from other providers.
If you haven't already, a good place to start is:
OU Engineering Web Site