Car keeps cutting out due to intermittent ignition switch however as it cuts out it for long enough it will continue if you hold the crank. Need to get somewhere 25 miles via motorway. Can I expect car to keep going with cranking as is needed or is this not a good idea/might it cause other problems?
This sounds like a spectacularly bad idea.
Depending on exactly how the ignition is behaving when you "hold it at the crank" it may or may not be doing unpleasant things mechanically - if it (for example) is actually keeping the starter motor engaged you'll burn through that pretty quickly.
But even putting that aside if you are holding the key in the crank position this is taking one of your hands away from driving - so you're not in full control of the car.
With the bonus prize that if your hand slips (say from hitting a bump) then the car might cut out. While you have one hand on the wheel. At motorway speeds.
So no, don't do this, it's a stupid and dangerous idea. If you need to get the car somewhere get it towed or trailer-ed.