an you still upgrade the stuff in the new imacs..?
I think the RAM is still user serviceable.
didn't know I needed the Starship comp
It's not (only) that.
A decently specced Mac will last you 5+ years generally. The better the configuration, the longer it will last. Consider the cost spread over the years.
8Gb of RAM is too little even for day to day usage sadly. Full stop.
You can order the machine with the minimum available, but you have to add it straight away. 32Gb at Crucial should set you off about 150gbp (against 600 at Apple).
You can do with 16Gb and upgrade to more later on.
Fusion drive is ok. I worked off one of these for several years.
Just get a large enough drive = 1TB.
Just the software and the apps will eat a lot of space. You will have a lot of local caches (unless you configure your setup properly) that normally eat a lot of space.
Consider that you always want to have about 20/30% of the drive free.
i5 processor is bottom of the line. False economy. Especially if you want to do 4k video. Get an i7 at least if you can, it will serve you for longer.
the Macs i"ve looked at is i5s and only 8Gb RAM, and 1TB Fusion hard drive.
Would this slow down the edit so much it would be unusable?
Depends on how much you are doing.
But it is not going to be very fast. Main bottleneck being the RAM in that case.
The i5 in the iMacs you saw at Currys should be the i5-8500 6 core. Speedwise should be comparable to the (older) i7 4 core processor I have in my current machine from 2016. It is ok. But I don't think it's gonna last any longer.