Bring post-it notes.  So handy for making notes on maps and flyers.

And zip-loc baggies.  For everything from storing damp undies to packing crackers and other snacks.  Keep one in your purse or fanny pack at all times.
Never call it your “fanny pack” in public.  Trust me on this.  It means something totally different in England.  And not very polite.
Learn how to use the flashlight on your iPhone.  If you don’t have an iPhone, bring a small light of some kind.  Handy in dark places, like churches, or bathrooms at midnight, when you can’t find the switch – because it’s actually a pullcord hanging from the ceiling.
Keep several tissues in your purse at all times.  This climate makes your nose drip – and the day will come when there is no TP in your WC.  Inevitably.
Tesco (grocery store) sells a roll of good ginger snaps for only 25p.  So addictive.  Tesco is cheaper than Sainsbury’s.
Yes, they will bring you tap water in a restaurant for free.  And it doesn’t taste funny.  And good news – food is always served hot.  I think it’s a religion.  Warning, though:  beer is never cold.  That’s a religion too.
If traveling in spring or fall, bring slippers.  The floors are cold – COLD – because there are no heated basements.  And bring a hat.  And gloves.  They are tougher than we are, softened as we are by our central heating.
A hooded jacket is obligatory.  Rain and wind can happen at any time.
I recommend leggings and a lightweight fleece t-shirt for pajamas – something warm that can do double duty during the daytime if – no, when – necessary.
Get ready for some unusual experiences:  e.g. church bells in the evening because the bell ringers are having a practice.  Just wonderful.
And be aware that not all fish and chips are created equal.  But do try the curry sauce.  And the blood pudding (but only once).
If you join the National Trust, every place you see after that will be English Heritage, and vice versa; but if you mention that you belong to one, the other will often give you a discount.
There are public footpaths everywhere, but they don’t necessarily take you anywhere.  Go anyways.
See, do and enjoy all you can while in one place;  you never get the chance to come back because there is too much beckoning to you in the next place.
(Comments and further advice based on your own experience is welcome!)