For an intercalated degree the most important thing is to do something you are genuinely interested in. A year can be a pretty long time stuck doing something you hate. Some intercalations are geared pretty specifically towards certain specialties eg. Emergency Care at Peninsula, but most are more general. And even if your interests change dramatically, doing any intercalation will give you points on your FP and specialty applications.
With regards to what speciality you're interested in... Honestly, it will come over time. You can't really work out whether you like something until you've seen it in action. You will experience a lot of specialities over clinical and foundation years, and will get a good idea of what you're into. There are also taster weeks in foundation years that are designed to help you work this stuff out
A few things that might help you start thinking about it though:
Medicine or Surgery? This is a pretty big split, and you'll want to know which you're interested in by foundation. Most people don't find this bit difficult
Competitiveness - are you interested in slaving yourself to death getting into neurosurgery? If not, you can probably strike it off
Lifestyle - different specialities have different lifestyle in terms of unsociable hours and on-calls. Comsider Emergency medicine vs. dermatology. What kind of life do you want to lead?
Honestly, if nothing stands out hugely to you that's okay. Some people have an interest in a certain field from day one, but most don't. Most people I've spoken to went into their field as a result of a strongly positive experience working a certain job. For example an oncology consultant who teaches us did an oncology foundation post and found the patients were extremely grateful and friendly, and his consultant was extremely nice and supportive. So he stuck with it. It doesn't have to be anything deeper than that.
I don't really know what I want to do either. I'm currently interested in Emergency Medicine, but while it may be fun as a student, working the job is very different. I don't know whether it will suit me. Otherwise... definitely medicine, and interested in trying out psych and GP, but I honestly have no idea. I think it's good to keep your options open and not blinker yourself too much as well.