You're welcome!  I share your enjoyment looking at these trees, and it has become an addiction.. trying new species, buying nice pots, etc. I would guess almost everyone on this forum has had one or more trees die on them, so you're not alone!
Others can answer about your nearest nurseries, where to get soil, and best type of soil for your tree.
But I will say that autumn is not a good time of year to repot. Repotting bonsai involves pruning the roots, partly so they fit in the pot but mostly for long term health of the tree in a pot. The best time of year for that is spring just as the first new leaf buds start to swell, because all the sap stored over winter is pushing up through the tree and it will continue to push out a spring flush of new leaves even with half the root mass. Whereas in autumn, the tree is trying to get the last of the rays to make sugars in the leaves and store them downwards in the trunk and roots. So if you chop the roots in autumn you're removing a lot of energy stored so far this season and removing capacity to store more over the next month or so. My guess is that's what killed your previous bonsai, autumn repotting. Over-watering is almost impossible in granular soil unless you permanently keep the pot in a tray of water.
If you find your tree is suffering in autumn from being potbound then 'slip pot' it, so you put it in a bigger pot with the soil & roots untouched, but with new soil round the edges and bottom of the new pot.
For your elm, I think you can safely wait til spring and repot then. That will give you time over winter to watch Nigel Saunders Bonsai Zone on youtube! He does lots of repotting in mesmerising detail.
Chinese elm will survive no problem outdoors, better if they will get more sunlight. The rain is no problem. But strong wind can be a problem. And ice can harm some species' roots, but elm is quite hardy (so suited to Newcastle?!) so should be fine outside. But it wouldn't hurt to wrap up the pot on cold nights if freezing temperatures are expected. Don't water on winter evenings because if the extra water freezes it can crack the pot and worse burst cells in the roots.
If you decide to leave it indoors it will probably be fine this year but if it doesn't get cold in winter it won't get a rest period and over a few years may start to decline. Either way, try not to make sudden changes, if you do put it outside do it in phases to slowly get used to the colder nights.
Have fun!
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