We bought our first house in the US, and this was the best bit about buying and selling a house in the US. I am not sure why any seller in the UK thinks they need to be present for a viewing. Frankly, it's embarrassing as the owners try and steer you towards the "best" features. This has several effects: it makes you immediately suspicious about what the sellers are trying to cover up, it makes you question the personal taste of the owner, and it makes it more awkward to ask about certain questionable features of the property.
For example, when I was buying in the UK, I went through one viewing where the owner omitted to mention that the big stream a few metres away from his house had flooded at some point during the recent past. I knew it was a problem, but didn't challenge him on it. I wondered if the seller ever got away with it, so that a buyer would not know about the flooding.