The current driving test can be failed if you 'Fail to make good progress'. However it is not an RTA offence in its self, but it could be construed as driving without due care and attention.
Driving slowly is not likely to be an issue if you are the only vehicle on the road, but when others join, you should not unduly impede their progress but you must stay within the legal limits applicable to your vehicle.
There should not be a problem on multi lane carriageways provided the slow vehicles populate the nearside lane only. Faster vehicles can overtake in the off side lane.
Yes a slow vehicle does constitute a hazard, But then so does rain, a junction, ice, glaring headlamps etc. Hazards are everywhere and it is up to a driver encountering a hazard to drive accordingly to either avoid it or to manage the situation safely.
A hazard itself does not cause mayhem, bedlam or incidents, they are always the result of other drivers failing to manage the situation properly.
Don't blame slow vehicles for the inadequacies of human temperament. If you allow your frustration to affect your driving, then you should not be on the road.
Long Live Slow Drivers - but please speed up a bit.