Is it okay to focus the PhD Research Proposal in more than one topic? Some context here: I'm a physics student from Argentina about to get my MSc. I'm applying to a PhD in many universities of Europe and some of them (Oxford and Imperial College London, for example) ask for a Research Proposal.

I find it weird that I must commit to one specific topic before I even know anything about the area (I know some stuff, obviously, but I'd like to learn more before I choose one specific topic). This is different in the US Universities (where I've applied too) where you just apply for the PhD Position and then have one year to take courses and choose your group and research project.

Because of this, I thought that maybe it would be better if I write about two or three possible research topics within the area of the research proposal. Is this okay? To give further information: I want to apply to the Information Theory group of the Physics Department of Oxford and I'd like to write a proposal about Quantum Thermodynamics but also about information theory applications to fermionic systems and out-of-equilibibrium quantum dynamics. This three topics have nothing to do with one another but the group I'm applying to is working on them.
The function of the research proposal depends widely on the nature of the admissions process. If you are being directly hired by and supported by a professor's research group, then the proposal should be directed to the interests of the particular group. 

On the other hand, if the department handles admissions and assigns students to groups later on, then you do not need to focus your interests too narrowly. The big question is this: are your interests in the "hot" areas, or other areas within the department? If you're applying in the "hot" area, then you need to be more specific (since there's more competition); if you're doing less popular topics, then you can afford to be less specific.