Buy the model I have, the 16gb 512gb M1 Air at £1400 new (or ~£1100 new-ish on eBay).

Keep both for a few weeks, transfer your stuff over, then sell your 8gb 256gb M1 Air for ~£700.

1TB would be the minimum for me, especially spending that kind of money. Also I doubt I'd get £700 for my MBA, seeing as that's how much I brought it for (refurbished from Apple). When I was buying it originally, the prices on eBay were more along the lines of £600. Even still, I don't factor in the money I'd get back from selling my previous machine

As for my reasons, there's a few. First of all, particularly on higher priced items, a lot of the trouble with the price comes to having the full amount in the first place. I can't walk into an Apple Store £700 short but tell them "don't worry, I'll have the rest later".

Aside from that, actually selling the thing is no guarantee, especially not for £700. I remember going to buy a laptop on eBay over 3 years ago and I still see it up there now. It might not be as in demand as the MacBook would be, but the point holds. Even if it does sell, the question then is how long it takes before it does.

Based on that, I assume if I can only afford something if I sell something else, I can't actually afford it. There's just too many factors involved. If I can afford the thing and I sell the old thing after, great, that's a nice bonus.