Has anyone got any experience of SheetCNC or UK-DIY CNC?

[JAZZCNC]

I appreciate the advice, and saw especially JAZZCNC and the thread that was mentioned about building machines for people who try to cut corners leading to frustration, but just starting out I can't jusify the 10-15k+ budget of what seems like the start of reasonably capable machines just yet. So is there anything that I can do that may give me the best of both worlds - not a full sheet machine, but capable of cutting larger parts with multiple cuts, perhaps, that would not make me regret the investment straight away?

Well, you don't actually need to 10-15k to get a capable machine but you do need a little more than 5k for a large machine like an 8x4. But the question you really need to ask yourself is do you really need an 8x4.?
It's quite possible to cut 8x4 sheets on a 4x4 machine with a little creative cam work. You simply feed the sheet through the machine and using registration markers or dowels etc you can cut the same large part an 8x4 could.

Am I being completely unrealistic in what I am wanting to get for the rough budget that I am consideriing, and I should just give up on that idea and start a lot smaller, first of all? I don't mind putting work in to build a machine from partial kits and other parts, and I am willing to learn but don't really know where to start, given that I am glad I asked about the seemingly 'perfect' machine to a layman, there is always something that seems too good to be true (and usually is), so again thank you very much for steering me away from that decision!

Yes and No.
Yes, it's unrealistic to buy off the shelf an 8x4 machine that will be any good with only £5k, the correct parts alone for a machine this size would nearly come to that.

No, it's not unrealistic to build your own 8x4 with £5k, BUT it's only really possible if you have the experience and equipment plus know where to buy all the correct components from. It's very very easy to get it wrong and many who have tried failed or completely underestimated the work involved or as is often the case not done enough research which leads to buying the wrong components so the costs spiral out of control.

Forget kits, I don't know of any kits that are cheap and worth buying and nearly all those that are anywhere near halfway good usually end up costing the same or more than DIY building by the time you add all the other associated electronics, etc.
Some of the better off-the-shelf kits actually work out more expensive than having one built from someone like me.!

I am in the UK, so it would have to be something available over here, and most of what I see on the market is much more difficult (or almost impossible) to get or much more expensive over here, so appreciate the advice from that point of view. I am lucky to have quite a large space I can use, so that is why I was even thinking of an 8x4 machine in the first place. I see people saying that given the right machine imported from china that you could get a cast iron machine for around 4k, but I imagine that is a complete minefield and support would be pretty much none-existant, so would have to rely on more local troubleshooting. But I suspect that a lot of people will have had some very negative experiences there too, so again would like to hear from anyone who can tell me otherwise!

At the moment the shipping from China alone would cost £4k and yes it's a minefield with near-zero support and machines with electronics which are like a box of chocolates with no lid.!
Also, there are cast iron frames then there's Chinese cast iron frames, the latter not always being a good thing.!
The same is true for the steel-framed machines they produce, There is a thin-walled tube then there is a Chinese thin-walled tube.? until I worked on a Chinese machine I didn't know a large box section could be produced that thin.!