£300 per month is a good start, but probably a bit on the low end? PPL is anywhere from £8K to £12K and a 0-fATPL from a good and reputable school can cost a minimum of £50K. At £300 per month, that's just over 166 months or 13 year's worth of saving. At £500 per month, that'll still be at least 100 months or just over 8 years. You could probably save up half and take out a loan on the other half, but even at £500 per month, that's still at least 4 years of saving.
Not saying this to put out your fire; I'm looking at the exact same thing as well and I have the added responsibility of a family, a few £££ in loans, and in my mid-30s. I'm just saying that this is the harsh truth and if anyone out there has an answer, I'd love to know it! At the moment, I'm signing up with a few other nursing agencies to get more work and be able to save more and more each month.