If it failed on front brake imbalance that means that the maximum braking effort of one side was less than 70% of what the other managed, that's a pretty big imbalance. Certainly enough to be dangerous if you had to slam on in an emergency. For comparison if the imbalance is such that one side manages less than 50% of the other that's classified as the more serious type of failure (a "Dangerous" fault) and you aren't permitted to drive the car until it's repaired!
In the advisories they are all on there ... pads and disks?
The minimum brake pad thickness for the MOT is 1.5mm (or if it's hit the wear indicator where fitted), so from the fact they only advised on that rather than a straight up fail means they're over that at least. But it doesn't mean that they aren't heavily worn.
My DOT 4 bottle had been low for awhile .. not realising why my abs was flashing as I was breaking.
Low brake fluid level in the reservoir suggests either a) your pads are completely shot or b) you're losing fluid somewhere. Neither are particularly reassuring scenarios!
From the fact that they advised on the pads (and Insignias are known to give low brake fluid level warnings when the pads are heavily worn) I'd suggest the former is more likely. When the pads are heavily worn the caliper has to push them further to make them contact the disc (which is further compounded by worn discs) and this means pushing the fluid further and this can push it past the sensor making the system think it's low.
Could it just need a blast and harsh brake a few times to get the fluid through.
No, it won't make a blind bit of difference and I really, really don't recommend going out and doing some harsh stops in a car that's been reported as having substantially out of balance front brakes. That imbalance could cause it to pull sharply to one side under braking and that could end badly if there's something hard in the way (or very badly if it's something squishy).
I think you're looking at new pads and discs - hopefully that explains your "low" brake fluid level as well otherwise you're hunting for a brake fluid leak :(