UK law is odd - a vehicle can be in a state which is illegal on the road under construction & use laws (C&U) but if the item is not mentioned in the MOT test you'll get a valid MOT certificate.
However, strictly speaking it is NOT legal and a police or VOSA bod can pull you & punish you & your vehicle. If you have an accident, your insurance may be invalid and you're in court in short order.
How likely this is & what happens to you depends massively on the vehicle, the mod, and the attitude of you & the policeman who pulls you over.
A minor infringement of some trifling technicality may never even get noticed. Mods which are more obvious and/or obnoxious (insanely loud exhausts for example) will be more likely to be picked up.
Likewise if you don't take the p*** and are reasonable and decent when pulled over you often get given the benefit of the doubt. If you kick off about it your life can be made miserable...
I've been pulled in many countries in variously stupid / modded / borderline legal vehicles and mostly the cops just want to look at it, as long as you're cool with them they're happy.
To quote the Dutch cops as I rolled off the ferry:
"This is not legal, this is not legal, and this is not legal"
"It's legal in England"
"Oh, then this is a really cool car!"