Try floor sitting. It's not perfect in the summer months since you get less airflow around the legs, but it lets you move a little and stretch as you think which is very healthy. Dozens of postures possible. It can be done cheaply, cheaper than a "normal" desk setup:

1. Folding floor desk(ideally taller and wider than a breakfast tray so there is leg room)

2. Lap desk for peripherals. This makes computer interactions ergonomic, no more hunching or reaching.

3. Floor chair/backrest so you can recline and get some back support in long sessions.

Plus all of it can fold compactly so it works well in a limited footprint.

Thinking you need infinite money to go for a walk is peak HN.

No, it's peak UK, the chances of the weather being agreeable for me to go for even a 5 minute walk without it pouring down are not good for much of the year.

I live in the Pacific Northwest where it’s a similar climate. I know we don’t get much right here in the US but we have invented raincoats!