I've been to several weddings with a free bar but they tended to be one or all of (a) not in the UK, or couple not originally from the UK - I think it's more common in America and some other places (b) in a self-catered or non-typical venue, e.g. one the reception was held in the parents house/garden and therefore all booze had been brought in by the hosts, no option to buy your own (c) very very wealthy couple or families for whom it was a drop in the ocean compared to the overall costs of the wedding.
I agree that the vast majority of UK weddings don't have a free/open bar and people don't expect it, although usually there will be quite a lot of free booze, the standard is to provide welcome drink, wine with dinner and perhaps a certain amount of money behind the bar to start the party off, but at a certain point in the evening if you want more you pay your own way which seems totally fair to me.
I think to be honest a lot of Brits wouldn't be able to restrain themselves at a completely open bar and it could get quite messy! To illustrate, at a wedding at a very fancy, expensive hotel we went to a few years ago, my DH came back from the bar delighted and surprised to have found that it was a free/open bar and he hadn't been asked for his credit card, just his room number. We were surprised as the couple weren't that well off despite the fancy venue, but thought maybe one of their parents was paying or something. We proceeded to have quite a few cocktails and G&Ts over the course of the evening, no way would we normally have ordered as many if we were paying ourselves, but they were delicious and as we were staying at the hotel overnight we stayed up late and went to bed quite merry! DH was staggered to be presented with a huge bar bill the next morning when we dragged ourselves down hungover, turned out it wasn't an open bar at all (beer, wine and soft drinks were free/paid for by the B&G but everything else had to be paid for or could be charged to your room which is where the confusion came from, he clearly hadn't understood the bar man's explanation that he was starting a tab!), which made absolute sense to me, in hindsight it was a bit shameful that we expected the couple/family to pay for our session on the most expensive drinks in the house, DH was a bit red faced about the whole thing and rightly so...!