This is just a quick guide that is focused on everyday English that native speakers use in informal situations in Britain.
Mate - somewhat similar to 'dude' this is an informal way of referring to someone (it could be a close friend or a stranger) i.e. 'Alright mate, did you watch the football last night?'
knackered - this means to be absolutely exhausted. It comes from the word 'knackers' a slaughterhouse for working horses in the past. Some examples include : 'I worked a fourteen hour shift last night, I'm absolutely knackered'.
gagging - to be desperate for something i.e. 'I'm gagging for a drink, let's go to the pub'.
pint - In the UK pubs and bars still serve beer in this old measurement (568ml). So you can order a half (half a pint , 284 ml ) or a pint. i.e. 'Can I have a pint, please'.
squire - this really doesn't mean anything. It is sometimes used instead of mate. It is used to refer to people and generally has friendly overtones i.e. 'Can I get you a drink squire?'
chief - again this is a somewhat vague greeting similar to mate normally used by shopkeepers i.e. 'That will be £6.99, chief'. I suspect it is an in-joke amongst shopkeepers.
dead chuffed - this means to be really pleased or happy about something. i.e. 'I'm dead chuffed I passed my driving test'.
dead - informal way of saying 'very'. i.e. 'I'm dead tired'.
pissed - drunk i.e. 'he was so pissed last night he threw up his whole dinner!'
pissed off - angry or very annoyed i.e. 'works really been pissing me off lately!'
fiva - five pounds (£5)
tenna - ten pounds (£10)
Well there are many more everyday phrases but I hope this helps!