I think the way people respond to 'is everyone alright for a drink' type of comments usually relate to the context. The response is as important as the question.
So, if you've been part of a group all evening, buying a round earlier, or accepting drinks from everyone so far who has been buying drinks and this question is asked, it's perfectly fine to ask for something, however it's worded - this is becaue you've already bought for them, or will do shortly. If the latter, then 'oh yes, I'd like a.....and the the next round will be mine' is fine. People who've already bought drinks or will do so shortly don't usually feel awkward about having one from someone else, even if it's phrased as 'is everyone alright for a drink'.
People who might feel they should say they are fine and not ask for one are those who've just arrived and so who haven't been in the round buying group and who probably do t want to join it late in the day. So they might say no, as might someone who isn't planning to buy a round later.
As Op had already bought drinks, I think she could have taken the question as genuine and certainly felt comfortable in asking for something - and the BF would hardly have been surprised in that circumstance really.
Round buying can be tricky though. What if you go out with the plan to have a couple of drink and spend £10-15 and when you get somewhere it turns out to be round buying in a group of 6 or 8 - you might accept. First drink before you realise it's a big round group, then suddenly you feel committed to offer a round which could cost £40-50 even though you only plan to have a couple of drinks. It's difficult to always know what the culture is on arrival or avoid feeling irritated that you've ended up spending more than you wanted or others drank your drinks and didn't reciprocate. It's why unless drinking regularly with groups in an established pattern, I think it's usually easier to just buy for one or two or yourself and decline offers, especially from people you do t really know.
Perhaps the BF expected to go out and spend £15-20 on a couple of drinks for GF and himself - perhaps he doesn't usually buy rounds/socialise with people who do. Perhaps the rounds were really expensive and he was faced with a much bigger cost than he'd imagined for the night. That said, he should have cottoned on faster and not kept accepting if he wasn't willing to buy.
The point is, tHess things can be complicated. Just because you and your friends usually do one thing, doesn't mean anyone who does something different is rude, especially in a one-off meeting. If someone continued to accept drinks and never bought it would be odd - but I can see how on first time meeting people this could happen.