Is it possible to install Android on the BQ E4.5 UE and reinstall Ubuntu Touch later when some important apps are available?
How to do that?
Yes - it is possible to downgrade it to android and you can follow this guide - http://a25.co/ubuntu-phone-how-to-install-android/ (which is not written by me).
Some of the information there is a bit outdated (the latest android for the phone at this moment is Lollipop) but the basic principle is the same.
As for reverting back to Ubuntu Touch - it is possible but I haven't tried it yet and can't be of much help with that. I think the procedure would be the same as with the android install, just using the Ubuntu firmware provided at the BQ site instead.