Most unis will allow to to transfer from a BEng to an MEng at the end of second year (providing you’re averaging at a 2.1 or higher I think) and vice versa. BEng usually have lower requirements so it might be less stress
I had trouble deciding this when I was applying for university, and am not current on a masters course. While whether you progress to the final master year depends on the grades you get (at my university you need 58% average at the end of third year to progress but this can differ depending on the university), I'd recommend if you can applying for the masters. It always easy to move to the bachelors if you need to, however sometimes moving up to a masters can be a bit more difficult. Also if you're pretty sure you want to do the masters, it will save you from having to apply for it in the future.
Hope this helps,
Abi (3rd year electrical and electronic engineering student)