I'm thinking along the lines of running the small garage tools off the kitchen/downstairs ring main.
Then run a separate circuit off the new cu for heavy stuff like a welder, also to the garage, independently from everything else. I presume?
Why overcomplicate things? Just run a separate ring main round the garage. No need for anything to come off the kitchen ring then. No need for any other supply into the garage save for the lighting circuit, which is presumably already there.
If the welder is fed from this ring, it's powered straight from the consumer unit, so you will already minimise its' influence on other devices in the house.
So the welder can't inter fear with the circuit to other stuff on the downstairs ring main, like the cooker, but also the tv and home cinema stuff, presumably. And if she turns the cooker on, that won't mess with the welder either.
The violence of the welder draw was enough to dim and flash some lights I've run of a plug socket in the kitchen. The flash was caused each time the welder arked on, which is very rapid. I don't want common electrical house hold appliances exposed to that sort of current draw.
It can't be good for them, can it? I'm thinking better safe anyway.
Other items won't be
exposed to the current draw of the welder (which is no more than an electric kettle or fan heater in any case). All that is happening is that the mains voltage is dropping slightly due to the resistance of the supply, so it's not a case that it won't be good for anything.. just noticeable. The fact is, loads like a kettle or a fan heater are generally turned on and off less frequently than a welding arc, so their effect on the voltage is not noticed.
If the welder was previously fed from a lighting circuit as implied, I'm very surprised that the fuse didn't blow, and not at all surprised that the effect was noticeable, especially to anything fed from that same lighting circuit.
Even with the detailed updates to the wiring, though, it's still possible that you'll get a little flickering of the lights when welding due to the resistance of the incoming supply. There's not a lot you can do about that, though.