I think part of the British/English/etc. thing that is interesting to me is accents. Generally you figure out where someone is from because of an accent, but British accents seem like they could be tricky to pinpoint if you're not used to them and so that could be why many USians default to 'British' instead of the specific country.
Scottish and Irish accents are very distinct from a Southern-English accent, but Scottish and Irish aren't all that distinct from each other. However, from what I gather, all of those accents are heard with some frequency in the US (immigration, media, etc.) and so many USians can tell the difference. Welsh accents and any English accents from the Midlands up, however, aren't all that common but if you hear one you can figure out that the person is British.
This is all just my take on things and not actually being American I have no idea if my assumptions are correct! ETA: Obviously it's not just Britain that has this 'issue'. Most of the time I can't tell the difference between an American and Canadian accent. I usually can, however, differentiate a NZ accent from an Oz one.
As for the dining out at different times bit, I would agree with you there. I wouldn't expect to see children (young children especially) out for dinner after about 7-ish either. Many pubs, bars and restaurants here seem to change to a more adults-only ambience after about 7/8 o'clock by turning lights down and music up.