Saying "I'll have an ale" would be like walking into a restaurant I'll have chicken. How would you like it cooked? From what I've heard, your pubs seem to only serve one or two brands, so you might not have much choice.   Here in the U.S. if you ask for an "ale", you would need to qualify it - do you want a bitter, a pale ale, a mild ale, an IPA, a stout, a porter, barley wine, etc.

I've never known anyone here in the UK walk into a pub and order "an ale" - as you say, that would be pretty pointless.

It would be the same as you describe for the US - you would have to be more specific and say "a pint of bitter" or whatever. Some pubs may only stock one type of bitter so that would work. But if the pub stocks more than one bitter, you'd have to be more specific and say for example "a pint of Adnams bitter" or "a pint of Wherry" or whatever.

I think we're saying the same thing here?