Only main English-ism (apart from "chips" and "crisps") I'm aware of that people may not know is "quid", which is the slang for pound (as in Pound Sterling). 33 pounds and 10 pence can be written as ₤33.10p, and can be read as "thirty-three pounds and ten pee".
There's also "loo" instead of "dunny".... but of course "toilet" is just fine universally

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When handing over paper currency, especially 20 pound notes, don't be surprised if merchants hold them up to check them for counterfeiting (especially in pubs).
On that note (pun intended), you will most commonly be handling lots and lots of 20 pound notes. No jokes - especially if you draw money out of an ATM.
You will build up a lot of change, especially if you're paying for small things with cash, using the Oyster machine, etc. etc. Pound coins are useful, but use them often to prevent carrying a burdensome change purse.
Standard measure of a beer in pubs usually starts with a pint (not smaller than that, cf. in Australian pubs). Yes you can get Fosters in UK; I do not advise drinking it, unless you're cheap.
There's a whole bunch of dishes which have odd names in England, but I don't see them often on pub menus, so no biggie there.
In Central London, it is more often than not quicker to walk between two adjacent tube stations (or even non-adjacent ones) rather than to hop on the tube.
Many restaurants will add a "12.5% Optional Service Charge" or otherwise known as a tip. It is auto-added to bills and can be seen on the list of charges. If you do not want to pay it, request to have it removed (and expect a death stare from the poor sod who will have to reprocess your bill as such). Probably should leave it on and pay it unless you thought the service was really that bad (though I realise some people have a moral etc. objection).
Do not attempt to annoy or generate a reaction out of one of those royal guards with the tall fluffy hat things. It's amusing to note how they can't even crack a smile, but try anything funny and you might as well be wearing an outfit covered in bulls eyes targets. No jokes.
If you walk down the street and think for a moment that you're back in Australia, don't be surprised, as there are lots of Aussie expats in London.
Like
Mal, don't tempt pickpockets, especially when catching transport (tube, tube stations). Keep an eye on all bags, and keep a keen eye and touchy-contact with handbags.
Well, that's my brain dump for now.
I actually didn't know that "pants" referred to underwear in England.

I know that "undies" is not common in England compared to Australia, whereas the term "knickers" is much more common in Old Blighty.
I generally use Wikivoyage (I used to use Wikitravel, but now I use the CC fork, Wikivoyage), which includes sections on language and customs. The sections on what to eat and drink is also very useful.
United Kingdom
London