Apologies - I realised once I'd typed this that it's long. But it is focused solely on lighting solutions and issues around light.
Yes, candles (and matches to light them) can be a cheap and relatively easy to find option for light in the event of power outtages. But they need safety measures. So if candles are something that you are used to anyway and can use safely, have proper holders to keep them secure and not get knocked over, remembering to never leave alone or let them burn too low, keep away from draughts/pets/small DCs etc - then it is less dangerous.
But if you have the money for torches, (and I agree with Bellini that torches or LEDs are generally brighter lights than candles), they are a far better idea.
A mix of battery operated (and a stash of correct sized batteries) and rechargable ones is a good idea. Power banks that you might use to recharge your phones when travelling etc are useful for recharging phones and/or USB-rechargable lanterns. Camping solar panels can recharge those banks in the daytime if the power is still off. Decathlon is out of the 2 panel versions but still has the 3 panel version for £50 on the UK website, and other camping stores are likely to be selling off any they have as the end of season sales happen. You can also get solar rechargable lights that can recharge in daylight rather than needing to be plugged in to the wall/powerbank.
Other than camping and outdoor stores for end of season sales, look in places like Lidl and Aldi middle aisles - I got a really good rechargeable LED worklamp in the tools part a couple of weeks ago, which is bright enough to prep and cook food by which was under €20 (I think €12.99 but can't remember exactly). DIY stores are often also good places to look - garden solar lights, work torches etc.
Even if you are using candles, only use them in places you will be spending a long time in. Kitchen, dining room, living room etc. And blow out each time the last responsible adult is leaving that room. But if you can have lanterns, that is easier.
For places like at the top and bottom of the stairs, corridors that people have to go along, bathrooms, basically places where people will move through or have short visits and need to do it safely, definitely have battery operated or rechargable lanterns that are bright enough. NOT candles. Bellini's milk-jug lantern is very useful for those kinds of spaces.
And also think about those spaces where people will be moving around - don't have clutter in hallways, make it easy to find what will be needed but also have it safe to move around and not trip over things in low light levels.
The other thing to think about, if it is feasible in your house, is to get organised while it is still bright outside - to make it easier and safer with lower than normal light levels once it's dark outside. Sort the laundry off the clothes horse into a basket out of the way. Prep the food for cooking later, or even cook it and keep it warm for later. Get any washing up of dishes already built up done out of the way, so all you will have are the dinner things not all day's washing up. Have the beds made and bedroom floors cleared of toys ready for bedtime (have lots of fun in daytime, just put all or most of them away as it is getting dusky). That sort of thing.