How should I make speculative applications in the UK?

I'm in my third year of a five year degree in the UK. I need finance experience this summer to have a realistic chance of getting any sort of good summer internship. What is the best way to approach small companies, especially ones that barely even mention careers on their websites? I'm not asking about the content of my CV and letter, I just mean:

Should I phone or email first? Are all emails to HR or the General Enquiries email address hopeless? If I get no reply for my first email, what do I say in the follow up?

I think it has been said before that networking is different/non-existent in the UK.