Having checked out that site, I think, at the risk of being called a Provincial, that most of that advice just applies to Americans visiting London, & even so, is well out of date & a bit demeaning even so.
The stuff about shorts & trainers is rubbish really, as people here wear that all the time, even in winter, including our preposterously overpaid postmen, & anyone carrying an umbrella in case it rains is just a wazzock, or Japanese. What's the worst that can happen? you get a bit wet, right, but you'll soon dry. You simply can't dress worse than the average British person, no matter how hard you try. Believe me: I am one & I live here, & I'm slightly stylish by our standards.
"minicabs" is a purely London term, & non-licensed foreign-run cabs are allowed there & are pretty dodgy, as I've found, but London's a big place & has its own internal laws designed to protect the ultra high charges levied by their black cab drivers, all of whom are scoundrels. Elsewhere in the country, ALL taxi charges are set by law by the local Council & clearly displayed & are fair, & we don't have the London Mafia.
Tipping. Outside London, & especially in the "Provinces", giving extra money for something you've already paid for is simply looked on as ludicrous. Unless the stuff you got seems extraordinarily cheap & good, even by our standards, & you're drunk or just like paying more for stuff where the price has been clearly indicated, in which case, Hell, why not? The recipient will be delighted, no doubt. But it's entirely undeserved, as we have a legal minimum wage of £ 5.80 per hour ($10.00), which is reasonable & not made up of tips. Never tip in a pub, as the other people at the bar/table will just steal it.
A dazzling American smile, a heartfelt thankyou & a nice reserved handshake is all the tip someone proud of their service would want, as observed on the other website, is an appropriate tip, in our view.
Portions of food in the UK are Euro-standard, agreed. Deal with it. Buy the meal twice if you're still hungry, but you'll end up like the fat Sheriff in 'Race with the Devil'.
Outside London, everyone you meet will be delighted to see you & help you, especially if you're standing there puzzling over a map in the street. No tip required. Do not use your IPhones on Googlemaps in public in London, however, as maps are cheap but iPhones are dear. Common sense, really.
I think, finally, the one thing that my acquaintances would agree on, after extensive research vis-vis Americans is: Lower your Voice Volume by approx. 30% when conversing amongst yourselves or in public in Britain. We have fairly good hearing here & only chavs, Liverpudlians & pikeys talk loudly, to compensate for their lack of status.
I may be bonkers but at least I'm British.