I'm using a 16" Macbook Pro - the screen is lovely, but too small for all day working so I sometimes hook that up to an old HP non-HD 24" monitor - not great resolution but does the job, and the Macbook Pro is fast - despite being an 'old' Intel! You'd certainly be fine with an M1.
A 16/256 24" iMac will cost £1649 - and the 16/256 (refurb) Mini's £759. Throw in a reasonable 27" monitor for £300 and I'm saving almost £600 (I already have a keyboard and mouse). I could get the Mini new, and still save over £400. The external monitor won't be as good as the iMac, but it'll be bigger. And then if in a year or two they beef up the Mini's and I want to upgrade, it'll only be the cost of the Mini itself.
I'm interested in the iMac reviews when they arrive, and it's appealing to get an all-in-one without the hassles of setting up an external monitor, and just discovered the trackpad is only a £50 upgrade (I've already got a Magic Mouse). Neck and neck between the two options at the moment.