Consider where else you can go that might be warmer/have power ie. work / school / someone else's house.
Consider insulation (loft, walls, under floor) + draft proofing + fitting thermal lined curtains + maybe replacing old windows with modern, more efficient ones.
Talk to your heating engineer / plumber re any pipes that should be insulated.
Make sure at least one bathroom has a window [easier to go to the loo when there's no power if there's natural light]
Have a gas hob fitted if possible [works with no electricity]. Buy an old fashioned kettle that goes on the hob - easier / safer to pour hot water out of this than out of a saucepan into hot water bottles / mugs etc. Buy some insulated flasks to keep hot water warm for making drinks and some insulated mugs with lids.
Have battery powered smoke alarms / a carbon monoxide alarm.
Make sure you have at least a couple of battery powered alarm clocks / normal clocks / watches [you're probably going to want to deliberately re-jig your day to wake up at sunrise to make the most of the daylight hours]
Make sure you have battery powered / mechanical versions of things like a landline phone that plugs directly into the phone line and doesn't need electricity, calculator, tin opener, jar opener, torches (don't rely on candles) [ideally everybody who isn't a baby should have their own head torch so they can see what they're doing whilst being able to use both hands], radios, door bell, kitchen scales, whisk, a couple of sharp knives (if you'd usually use a food processor).
Work out the easiest places in your home to keep warm and spent most/all of your time there. [Think about this if you're planning to make a mostly open-plan living area] Consider a gas powered heater ie. a Superser that can be used indoors.
Consider moving the furniture around / to other rooms and pitching a large tent in the space - if you're spending a lot of time there you need a bit of room to stretch and move around.
Remember layering - underwear, thermal underwear [I'd wear typical undewear underneath thermal stuff, as if there's no power the only way to wash stuff will be by hand, so thermal long johns/leggings/vests etc will need to be worn for multiple days. The adventure / sporty guidance about only wearing your special thermal underwear without anything underneath isn't really applicable when surviving with no power] then long sleeved tops, jumpers, leggings, trousers, a couple of pairs of socks. I wouldn't wear a dressing gown or similar as too difficult to move and do stuff in, but stick with a number of T shirt weight tops under a couple of jumpers. Consider a beanie type hat - we lose heat from our heads. Consider scarves. Definitely wear gloves - thin ones underneath thicker ones - you want a combination that allows you to use your hands most of the time, even if you wear mittens to sleep. Consider a balaclava or similar to cover your face.
Remember you need to insulate underneath yourself - warm air rises, so floor level is likely to be coolest. If you're sleeping somewhere other than your own bed, consider how you warm the space beneath you eg. move your mattress / use a camping mat / sleep on a duvet etc. Then consider adding extra layers of duvet / blankets etc on top. Consider using a sleeping bag. Wear full length pyjamas to bed, socks and a hat. [Again, any advice to wear v little in a sleeping bag for campers isn't really applicable here, you've got no way to wash your sleeping bag] But definitely don't get into bed wearing wet clothes. Use a hot water bottle or two / bring your next day's clothes into bed with you to warm them.
Consider a camping stove for heating water and food, if you've not got a gas hob. Be wary that some gas stoves cannot safely be used indoors due to carbon monoxide. Hot drinks and food will taste good. If you can cook, you might be able to save some of what's in the freezer before it goes off.
Consider ways of charging mobile phones - power packs that are charged on electricity help a little, running them from the car helps sometimes. Not sure there's a good answer to this other than running a generator which isn't a usual thing in the UK.
Consider baby wipes/equivalents for washing people [to avoid heating more water than essential] Don't let anybody get extra cold by washing. Do strip washes in warmed areas, with warm towels available. Use lots of deodorant. Consider vaseline to protect lips and the edge of nostrils. Have some emollient cream available for hands.
Disposable crockery is maybe worth doing, but it's hard to buy and store in large quantities, so I probably wouldn't bother.
Consider entertainment / hobbies - books to read, card decks, board games - avoid the stuff with fiddly tiny pieces as unpleasant for cold hands.