Don't want to put a damper on things but your £21,000 estimate is a bit low.
Don't forget,
Hours building c£90 per hour in the UK
CAA exam fees £750 plus 50 odd quid per resit (if you need any)
Multi Engine Piston course £1500
CPL and IR you have allowed for but there is the possibility that you may not do it in the minimum hours. IR training is about £300 per hour plus approach and landing fees.
CPL and IR test fees £607 plus aircraft hire. Try to make sure you pass first time
B & B accommodation while you train
You are right to be cautious and should consider the financial implications of all this. As for me, yes I'm one of those in debt but don't regret a moment of it. The training is an adventure, difficult at times but worth it in the end!