In American English, you usually say 'Hey guys' and 'Hey dude'.
I'd like to know the equivalent in British English.
Thank you in advance.
We need to make the distinction between words which mean the same as 'guy' - in other words, informal alternatives to 'man' - and words which you use as a form of address or greeting, as in 'Hi guys.'
There are many British words meaning 'man', such as chap (old-fashioned upper-class connotations) , fellow (likewise), fella (informal equivalent), bloke (more working-class connotations), geezer (likewise, mainly London), or lad (late teens/early twenties or 'the lads' for one's social circle/drinking partners). I'm not sure where Adrian learnt the word 'cove', but that is NOT in current usage. The vast majority of British people have never heard of this word, and the few who have would fall off their chairs laughing if you tried to use it.
However, none of these corresponds to the American word 'guy' as in 'Hi guys', because they are not generally used in greetings. 'Mate', meaning friend, is used as a form of address, and I agree with Paul that 'Alright mate?' is the universal informal greeting among men. There is no female equivalent, and no gender-neutral term. The 'mate' term of address is exclusively male-male.
There is no plural equivalent, either. It's not usual to say 'Hey mates'. In fact, many British people do say 'guys' now. While 'Hey dude' would sound silly in a British context, 'Hi guys' is commonplace, especially among younger people.
'Hey guys' is used here. As is 'hey man.' 'Dude' not so much, it sounds a bit silly if someone with a British accent says it.
What you are looking for is probably 'alright mate.' Ubiquituous when a man greets a (male) friend of his.
'Chap' is a bit twee, although it is a perfectly valid word. It's not a word I would use myself too often but anyway. 'Bloke' is just another word for 'man', but it wouldn't be used in a greeting which is what you seem to want. No one would ever say 'hey blokes' or 'alright blokes', it isn't used like that. Another one would be 'fella', which would be a dialectal variant of 'fellow.' Again, not so much in greetings (although it is possible to say 'hey fellas') but as a more general word to refer to a male.
I thought of you, Hamed, as I was entering the underground at Victoria Station in London today. The entrance to the station was horrendously busy, and there was a London Transport employee shouting instructions to the crowds of people going down the stairs. 'Guys! Keep left!' she shouted. So there you are - if it's good enough for London Transport, it's good enough for me. 'Guys' is now official British usage. A formal announcement might still address us as 'Ladies and gentlemen..', but informally, 'guys' seems to have taken a firm hold on these shores.