Well if she doesn't deliver any customer service and talks to people in that manner then she is only making a rod for her own back and she is in the wrong job.I was at a beefeater (owned by the same company) with a friend and his wife the other week (wasn't my choice - they have a young one and wanted to play it safe with where we went) and that very much matched your first night experience. Empty restaurant except for us and the table next to us. I guess it makes sense from a staff point of view - only one section of the place they need to keep an eye on!
Given it was an Indian restaurant I assume it was an independent place (I've always been a little confused by how you don't really see chain Indian or chain Chinese places like you do with other foods like Pizza, Italian, etc etc) and I have noticed that in general the chains have been more heavy handed than smaller places. Maybe a mix of independent places being more reliant on getting as many people through the door (of course chains are too - but its a little less direct) and chains being a bit more wary of potential impact to their brand if they have a headline story of them breaking the regulations?
Not that this is a defense of poor service (and it sounds like the service you got was very poor and I certainly am not surprised that you don't want to go there again) - hospitality staff are currently being utterly shafted. Overworked, understaffed and underpaid, whilst at the same time probably now being the main group of workers outside of health and care workers who are most likely to end up with COVID (and thus either actually get ill, or at the very least have to self isolate). It isn't a surprise the poor woman didn't react well to you making a joke at her expense. Especially something along those lines which she probably has heard time and time again (such jokes stop being funny after the 100th time!).
How is she over worked? Working in a pub that is a third full.