Linux

I got myself an old EEE PC for exactly that purpose. (Except, substitute python with lua).

8h battery life, cost me €20 and does what it's supposed to. Just make sure you get one with an Atom N280 or better. The popular N270 is 32bit only, and more and more programs are dropping 32bit support. Some of them you can DIY compile for 32bit, some you really don't want to.

(For example, compiling Node on an Atom N270 takes around 3 days.)

I had one with an N270 first and replaced it with one with an N450 to get 64bit.

Maxed it out with 2GB RAM, a cheapo €10 SSD that maxes out SATA and overclocked it to 2GHz.

It's not fast by any stretch of the imagination, but it's totally ok for editing text files with Kate and compiling with platformio.

t480 or t470

A refurbished Thinkpad T480.

Refurbished Thinkpad. The answer is always refurbished Thinkpad.

Instead of just throwing random preferences out there, I'll help clarify the field of comments:

1. Thinkpads USED to be a safe choice, but Lenovo has been tainting that model line for a few years. Search and find specific models, and don't just buy because it has the Thinkpad brand.
2. Framework is 100% ready to go. They have a Refurb store where everything is cheap, but if you find one cheaper, get it.
3. Dell had a ton of Linux ready laptops under the XPS brand not long ago. Search and find out which to make sure, but they shipped with Linux installed.
4. I hate to say it, but HP Probooks were solid and shipped with Linux also. Terrible company, but they make decent enterprise products. They'll sell for cheap on eBay.

Under 140, latitude 5290 (from 2018, 8350u). I bought it ~2y ago. Small, 12 inch, decent keyboard, very upgradable, decent build quality, but the LCD is horrendous. Edit: used, could probably be found under 100€.

A refurb!

People are going to say Thinkpad but I am going to say 2013 to 2017 MacBook Air. Inexpensive. Light. Good looking. A joy to use. Faster than you think. And well supported under Linux (I use EndeavourOS on them myself).

Used Thinkpad X or T series

maybe search for system76 too. I found one on my local craigslist.

Do they sell ASUS in your area? I bought a 2nd hand 14" Vivobook with a Ryzen 5 GPU and it runs Fedora 42 like a champ. First laptop I've ever owned that I don't worry about overheating. It's been my chuck-it-in-a-backpack travel machine for two years with no problems.

Don’t know where you are from, but I got a MacBook Pro 2015 for 150 and it can be forced to the latest macOS or just any distro of your liking. I will say that it can get quite hot and it’s recommended to switch the cooling paste and clean the fan.

For a cheap device it works fine and in this price bracket a better screen is barely possible.

I do have a surface book 2 that I will sell between 150 and 200, but I think Linux support is finicky at best. I ran popos on it, but there are just a bunch of things which would work a lot better on other devices

Considering your budget of 200 GBP / 250USD, I would recommend laptops meant for school. There are plenty of refurbished laptops out there with a decent battery condition and overall state for sale around €100. Most of these machines aren't more powerful than most entry level Chromebooks and often have a Pentium or Celeron CPU, but that's a tradeoff you'll have to make. Another advantage is that they usually come with a touch screen and decent display, which is nice if you're out and about.

intel macbook air works good for me, with debian and xfce

The generic answer for this is to get a refurbished thinkpad. Pretty much any T-series fit your needs and there's plenty of pre-leased corporate machines around which are refurbished and often have even a some kind of warranty.

I have recently bought 2 dirt cheap thinkpads, one for me and one for the wife.

T490s - i5 intel
T14s - Ryzen 5 AMD

Both are tick all your requirements except for the numbpad, the T14s is definitely worth the extra money, though. It can even handle some medium gaming.
Both have upgradeable nvme ssds. However the ram is soldered on the "s" versions of these laptops so find one with 16gb or more.

A light Chromebook?