Yes. Done scratch test higher and lower on the trunk. So tree is dying from bottom up?

Bonsai Al wrote: Yes. Done scratch test higher and lower on the trunk. So tree is dying from bottom up?

That is correct. If the bottom is dead, which apparently it is, then it will not recover. There's nothing you can do.

Is there a way to find out what caused the base to die?

Bonsai Al wrote: Is there a way to find out what caused the base to die?

Not really, I can only guess.

You wrote it had green buds. It shouldn't have (assuming you're in a location where it is winter now).
Can't tell why the tree doesn't have leaves. It may have gone dormant, but that seems unlikely. Probable cause is improper care like underwatering of overwatering, wrong location, wrong climate (low humidity, too high temperatures).

Thing is that elms belong outdoors (and this is written as an answer to posts on this forum multiple times per day).
Leaf drop would be normal in that case. The tree would have formed buds - for next year - that would have leafed out in spring.
They would not have been green yet.

This is what winter buds should look like:

I live in the UK so it is colder weather now. Winter. Probably a few months ago it shed leaves which I associated with autumn. The tree kept on the bedroom windowsill so it gets good light. After the tree shed leaves i looked up getting life back to the tree so would use the submerge pot in water for 10 minutes to get water to the root mound. Got a good spread of green buds. The buds are no longer green. Maybe due to winter hitting?

Ive scratched one of the early lower scratch test spots more and under the brown in the light white/yellow colour. Is this a sign of life?

Im really confused. This hobby is turning out to be more difficult than I anticipated.

Bonsai Al wrote: I live in the UK so it is colder weather now. Winter.

Same weather as in the Netherlands. My chinese elms are outside, where they belong,doing fine.

The buds are no longer green. Maybe due to winter hitting?

They are no longer green because they died.

Im really confused. This hobby is turning out to be more difficult than I anticipated.

It is a very hard hobby if you have no horticultural knowledge at all.

Bonsai Al wrote: I would use the submerge pot in water for 10 minutes to get water to the root...

Makes me wonder... I don't like or use this technique.

Bonsai Al wrote: Im really confused. This hobby is turning out to be more difficult than I anticipated.

Yes, it is. Growing bonsai indoors in the UK can be very challenging indeed. Don't be confused though, read as much as you can about your environment and trees suitable to grow there, outside. There are many.

Sadly growing the tree outside isnt viable for me. I may try make a humidity tent. Can build a frame and some cling film around it to see if i get life back. Im trying desperately to be optimistic.

Bonsai Al wrote: Sadly growing the tree outside isnt viable for me. I may try make a humidity tent. Im trying desperately to be optimistic.

Yes, do that.
If there is hope you should try everything possible, and please let us know if it worked after all. Good luck!

Sorry to say but.. This tree is gone