Probably invalidate insurance, as another offence and also potential charges from any accidents caused itself, ignorance if claimed would not be an excuse in law and being a total t**t for doing this knowingly...
Surprisingly, it wouldn't invalidate insurance, though being convicted of it would certainly push your premiums up the next time. And repeated offences of that type are could result in most insurers refusing to insure you at all.
For the legally required third party insurance, I don't think companies are allowed to invalidate it because of the vehicle being unroadworthy - even driving when the MOT has expired or failed won't invalidate that in any policy I have seen.
Ignorance of the facts *can* be an excuse in law, but won't usually be in a situation where it is your responsibility to ensure the vehicle is safe to drive before doing so.
[EDIT]
What I forgot to mention is that it will also automatically incur a charge of Driving without due care & attention.
I know because it happened to me about 35 years ago (maybe a bit more) when I had bought a 2nd hand escort van, and hadn't checked the reservoirs - the brake fluid reservoir was dry, so the first time I braked hard it sucked air in, the next time I braked at all, they didn't work and I ran gently into the back of another car
The copper who came when the other guy called was actually very helpful. Although he (rightly) gave me a ticket, he also had a look and saw what the issue was, then drove me to the nearest garage to get a can of brake fluid.
But said ticket resulted in two charges - driving with defective brakes and driving without due care & attention, and when I queried it, I was told that one automatically followed as part of driving with appropriate care is ensuring your vehicle is roadworthy to start with.
My insurance did pay out on the third party claim, I only had third party fire & theft on the van, as it only cost £40.