Not sure about the hard shoulder question, but the emergency services can't 'force' you to make a dangerous manoeuvre. You are in control of your vehicle, you pull out of the way when and where it is safe to do so.

I assume emergency vehicles don't use the hard shoulder because it's a refuge area and may have a vehicle on it. If the traffic on the motorway had come to a complete stop then I'd expect they would use it.

I'm familiar with that stretch of the M25 and can appreciate how busy it gets, so I assume another reason why the shoulder isn't used is because some dim-witted driver may suddenly exit the motorway right into the path of the emergency vehicle.

There are also brief stretches where there is no shoulder.

I wouldn't want to be doing 100mph on the hard shoulder and I don't think I've ever seen an emergency vehicle doing that. Usually they are in the outermost lanes.

Nobody expects to be undertaken on the hard shoulder. It's also more likely to have debris on it.

>>hard shoulder exists for this very purpose

I don't think that is the case - it's for stopping in an emergency isn't it?

Not every blues and twos is an emergency. apparently it is part of driver training to do a long trip with the lot on. Bloke I know did a trip from London up the M1, think it was somewhere in Yorkshire.