My ultrawide is eight years old now? Love it.
For work, it gives you the equivalent of two old skool 4:3 monitors side by side. It's also the right aspect ratio for films, for big budget widescreen, they use 2.35 to 1. In practise, that means 3440x1440 monitors show all the content of a 4k rip, but without the black bars. 29.5" 2560 x 1080 is also fine. Whatever you do, don't buy a 34" 2560x1080p monitor. Stick with the smaller size or get the extra resolution. Start at 240 quid for 34" big beasties.
If you decide to get a normal 16:9, I'd still push you to get a 2560x1440 monitor, if you are going above 24 inches. 27" 2560x1440 is a very sensible option. £160 and above.
Think of the 24" 1080p monitor as the bare essential. It's ok. A teeny bit rubbish, because it's a common resolution on laptops. You will see the pixels. But it's totally usable. £90 and up.
4k is indeed 4 times as many pixels as a 1080p monitor, it's a bit lost on me at 27" inches, but works out is best for above 27 inches in my book.
Quick way to think about these screen sizes and resolutions; take that 24" 1080p monitor. Add an inch and a bit to each side. You now have a 29" 2560x1080 monitor.
The 27" 1440 monitor is not only quite a bit bigger, but it has 1.8 times as many pixels per square inch. Add three inches to either side, now it's a 34" 1440p ultrawide.
Finally, a 32" 4k has the exact same number of pixels per square inch as the 1440p monitors but is as wide as the 34" ultrawide, and 3 inches or so taller. The other thing with 4k, make sure you have either a Display port or HDMI 2.1 connector on the laptop and monitor, or you will be restricted to 30fps, which sucks pretty badly.
Brandwise, you can't go too far wrong on Amazon, but I'd see AOC as the entry level, and then Iiyama, before you end up at more recognisable names like LG, Dell, Philips and so on.