In Britain...is it rude for a fuel station attendant to ask "Hey, hows it going man?" to a customer ?

First of all, they ask you for the ID every time because they see many customers and it will be impossible to remember the age of all of them. If they sell beer to an under 18 person, they will get a fine and a criminal record, maybe you don't know this. I know because I was a checkout operator. Regarding "Hey, hows it going man"... come on, don't be so posh. Live and let live. These are not the problems in life.