No, I think it's perfectly legal to own a flare (even parachute flares). However they are a maritime safety device, and you can be prosecuted for letting them off when it's not an emergency situation.
So, I suppose no harm in sourcing and keeping one in your car... but in the UK they can only be used for maritime or mountain emergencies (so if you're nearer to a mountain than navigable water, you'll get a helicopter... in theory).
Having said that, there doesn't seem to be anything in the highway code that covers it; only maritime, so maybe this is not correct.