>>> it is perfectly acceptable for me to go for a meal with others in the pub
No, it isn't. If the 'substantial meal' rule applies (which, after all, is what your post is all about) you must be in a 'Tier 2' area, where mixing indoors with anyone who isn't part of your household bubble is prohibited (including in pubs serving meals):
https://www.gov.uk/guidance/tier-2-high-alert#visiting-venues-including-shops-restaurants-pubs-places-of-worship-and-public-buildings
I had lunch in a Wetherspoon's pub in Norwich yesterday, where things were very different to a 'normal' Saturday afternoon. The guy on the door was being very strict about ensuring that nobody sat at a table with others from outside their own household bubble (although, obviously, he had to take a certain amount of information on trust) and making sure that everyone was aware that they couldn't order further drinks once they'd finished eating their meal and would then be required to leave.
Apart from anything else, the existence of such rules kept the numbers in the pub to a manageable level, with everyone able to follow the one-way system and keep well away from everyone outside of their household bubble. . Having been in that pub many times before, I know that such social distancing simply wouldn't have been possible on a 'normal' Saturday, even if I'd sat at a table on my own. The Tier 2 rules meant that Wetherspoon's could still get some of their normal trade while, at the same time, people like me could also enjoy their lunch with a couple of pints. It all seemed perfectly sensible to me!