Unless you're in a super fancy situation on your gap year in the UK - you'll be fine to call people by the first names. So long as the other person doesn't introduce themselves as a 'Mrs' or 'Mr' that is. It's quite common for the 'politer' among the UK population to skirt around a question or something they want to say - rather than just come out and say it. The United Kingdom are well known for being very indirect. They're a sensitive lot and asking questions directly can sometimes be seen as overly abrupt and rude.
Say 'Pardon?' instead of 'What?' and understand that Brits like to apologise a lot. If you stood on their toe on the train they'd most likely say sorry too.
'You alright?' - you'll hear this a lot as a greeting in the UK, but it's actually very unlikely the person posing the question will expect an answer. Just say it back. 'Hi' is another popular greeting - if in doubt just repeat. Handshakes, hugs and kisses are all used randomly too - in fact greeting people in the UK is a minefield as each person will have their preferred ways.
People in the UK like their personal space. Go into a cinema or board a bus and the polite thing to do is to sit as far away from the person as possible. Don't go and sit next to them - although if you really have to you'll be better off in the north of England than in the south.
Small talk by people in the service industry is common. Become an expert at this and you'll be a hero. Weather and sport are the two favoured subjects, but be ready for anything.