Hiya, Gabi, and welcome!
Missy, if you're going out with 30 co-workers, it is v. unlikely that you will be expected to "stand a round" of 30 drinks (and not even the hardest of drinkers could get through thirty drinks in a sitting!) You will probably find that the group will resolve itself into several groups of 4 to 5 people, each of whom will buy a round for their group. Or you may find that someone will call out "let's have a kitty!" In this case everyone puts an agreed sum, say a fiver, into a pot (and it's often exactly that - a beer mug from behind the bar). The kitty is then given to the bar staff, and whenever someone wants a drink, they just go up to the bar, order it, and the bar person removes the appropriate amount from the kitty and puts it into the pub till. If you're staying for a long session, the kitty may be refreshed from time to time by an additional "whip-round", i.e., another collection. If at the end of the session there is any money left in the kitty, it's often used to buy the bar person a drink, or put into one of the charity boxes on the bar.
It's quite okay, if you don't intend to stay long, to announce "it's okay, I'll look after myself" when someone offers to buy you a drink - in which case you're sort of establishing a round of one for yourself.
If you're going to the pub with co-workers and your boss, you may well find that the boss buys the first round for everyone, as a way of saying thanks to his/her staff. This does not confer an automatic obligation to buy him/her one back. If someone is having a birthday, they may well buy a drink for everyone in the same way. It's a bit like being the host of the party. Or, it may well be, if the birthday celebrant is young/junior/new to the workplace, that everyone else will offer to buy him/her a drink. If the group is large, obviously he/she will not want to accept every offer, and will decline with some such words as "thanks, but I'm okay for the moment!"
Like so many other things in Britain (including the constitution), pub etiquette is largely established by convention, custom and practice, and is unwritten. It could be that what I've described above doesn't apply in your case! Why not explain to your group that you're not au fait with Brit pub customs, and ask them to tell you their understanding of them. This will probably take all evening!
And do tell us how it went!
