Laptops are sold with a list of numbers, almost none of which tells you what you want to know. Here is what actually decides whether the thing will still feel quick in four years.
The one that matters most
An SSD, not a hard disk. If you take nothing else from this, take that.
It is the difference between a laptop that is ready in ten seconds and one that takes two minutes and grinds whenever you open anything. It matters more than the processor, more than everything else combined, for how the machine feels.
Nearly everything sold new now has one. Nearly everything sold cheaply second hand does not. If a listing says HDD or gives a large number like 1TB at a suspiciously low price, that is the old sort. Walk away.
Memory
Memory, or RAM, decides how much you can have open before it starts labouring.
8GB is the floor. Do not buy 4GB, whatever the price. It will feel slow on the day it arrives.
16GB if you keep a lot of browser tabs open, which most people do without realising.
On many modern laptops the memory is soldered in and cannot be added later, so this is decided at purchase and never again.
The processor
The bit shops talk about most and the bit that matters least for ordinary use.
Anything current will handle email, banking, photographs and video calls without effort. You do not need the expensive one unless you are editing video or playing games.
The one thing to check is that it is recent. A cheap laptop with a processor from six years ago is a bad buy at any price.
The screen, which people underrate
You will look at it every day for years. Two things:
Full HD, described as 1920 x 1080. Anything less looks coarse and text is harder to read, which matters more as eyes get older.
Matte rather than glossy, if you have the choice. Glossy screens mirror the window behind you.
Thirteen inches is portable, fifteen is comfortable to read. If it lives on a desk, get fifteen.
What not to pay for
Extended warranties. Rarely worth it, and your rights under consumer law are better than people think.
Pre-installed security software. Windows comes with perfectly good protection. The trial that comes bundled exists to start charging you in three months.
Touchscreens, unless you know you want one. Most people who buy one never use it.
Setup services in the shop. Usually forty pounds for something that takes fifteen minutes.
A sensible answer for most people
An SSD, 8GB or 16GB of memory, a current processor, a Full HD screen. That comes in somewhere around four to six hundred pounds new and will last five or six years.
Below about three hundred you are usually buying something that will frustrate you within a year, which is a false economy rather than a bargain.
If you are about to buy, send me the link before you do. I will tell you whether it is a good buy for what you want it for, and I do not charge for that. I would much rather spend two minutes on a link than an hour later trying to make a bad laptop bearable.