In theory you can train the not-IC nurses to become one and replace the not-IC nurse with a fresh starter so you don't need to add it all together. Assumes training capacity as you say and that not-IC nurses want to work in IC, which I assume lots don't for loads of reasons.
Not sure in the UK, but in the Netherlands you need a nursing degree and then do two years on top to become an IC nurse.
And I assume people to train you during an internship, which is tricky if everyone is already ultra busy.