These are more city experiences, but mainly in London:
Don't tip waiters/waitresses unless their service warrants it. We have minimum wage in Britain, and service is usually included within the VAT charge when paying the bill unless it states otherwise in small print.
Don't stand on the left side of an escalator as you will block everyone who is in a hurry. If you are too lazy or unfit to walk up a few steps then stand on the right like other people seem to be doing. (be more observant)
Have bad hygiene and you intend to use public transport within commuting hours. If you are overweight and sweating like a pig, then just maybe walking or using a bike would do you some good!
Using headphones/earphones that leak out noise when listening to garbage that you call music on public transport.
If you are a European couple and it's summer please don't think it's okay to make-out in front of everyone in the middle of the street. We get that your women is hot, and your penis is so small that you need to validate yourself by showcasing her to everyone like she's a trophy.
If you are a German or American tourist DO NOT wear white gym socks with those ugly sandals. Flip flops are worn by millions in the U.S, yet we always get to have the ones that want to stink up their feet wearing thick socks with sandals!
Travel to London and all you ever buy are the cheapest tackiest tourist gift items and r̶o̶y̶a̶l̶ ̶f̶a̶m̶i̶l̶y̶ memorabilia. You go to Cuba you bring back cigars and liquor, Belgium or Paris chocolates, but London meh just some tacky mass produced Chinese manufactured ornaments. Whatever happened to spending your time on trips abroad so that you stumble upon an item that you fall in love with and will treasure your entire life, then pass down to your children? Why must tourists now be in a rush to see everything, do everything and then end up buying the cheapest nearest thing available - Oh this will do, it says London on it, job done!
England is not Italy! Do not bring your high-strung high-maintenance girlfriend to a venue and expect to be treated like a fellow gentleman. The English are reserved when it comes to their wealth and do not appreciate peacocking showoffs, it's very improper. Just because your precious lady is wearing some "bling" and your sports car is parked outside doesn't prove anything, not unless you're an Arab then you get the luxury treatment.
Do not get lost, because it will eventually lead to you walking aimlessly with a huge map up and down the same road asking aimless people to give you directions, who in many cases is always the unfortunate one that can't remember the difference between left and right.
For Americans:
Do not refer to things as British when you mean to say English, Irish, Scottish or Welsh, or UK. You'd only use British or Britain when describing something that is common for all citizens, such as the British monarchy, British tax payers, British ideals, British pound sterling.
Do not brand all biscuits as cookies, they are two different things!
Do not call all confectionery a candy. Lollipop, candy kane or candy floss is a candy, everything else is a confectionery sweet(y).
Do not visit England but then act as if the U.S is the only place on this planet. American tourists have a habit of making rude statements such as:
"Really, you don't have that here, OMG you're missing out, we make the best ___. It sucks that you don't have them here, ugh well what DO you have THEN.."
"Oh my you call that a what? It sounds so strange, you people really are strange".
"Do Britain dentists not offer teeth whitening? What, you don't floss? Lord no wonder so many of y'all teeth look dirty and crooked, thank goodness I don't live here I like to look good". —So you spend money to visit a different country, only then to spend the entire time ridiculing their peoples customs, culture and foundational beliefs meanwhile regurgitated very old untrue stereotypes, yet then expect to be treated with hospitality.
DO NOT complain about how poor your hotel experience is when it was your fault that you booked a room at a random business hotel (these types of accommodations are meant for quick business trip stays and meetings, not for family or romantic vacation stays no matter how much the rooms cost or the facilities they have). I personally like active busy hotels with families or couples enjoying themselves, rather than hotels which feel empty and dead most of the time.
If you are in Oxford or Cambridge definitely hire a bike. If you are in London, Birmingham or Manchester DO NOT ride a hire bike unless in a public park, or you should avoid busy or narrow roads. No matter how experienced of a rider you think you are, drivers in England dislike bikes and people have bike accidents all the time (though much of these are by stupid narcissists who think they are self-entitled to own the street because they are on a bike and don't adhere to the highway code).