This is modern Britain, languages change. It's what people say and what they mean that counts - as long as the message is transmitted clearly and understood accurately then that's all that really matters. There's no use in getting worked up about it, I think you'll find that the Britain of the past where we all spoke perfect English is a piece of patchwork idealism. The BBC may have used received pronunciation and accurate grammar in the 1950s, for example, but the rest of the country was not.
"I was stood on" and "I was sat" is more true to how people were speaking in East London, Liverpool, Manchester and across the country even as far back as the turn of the 20th century. Regarding "I'm good", it's just a fashionable thing to say at the moment, that's all. People *know* it's not correct grammar but it sounds cool to the youth on the street, that's all.