with these 21 day FAA PPL adverts for £5k ... I guess it's pretty inviting. I know 21 days might not be achievable
The only thing inviting about that is that it pleases your boss.
Any 21 day PPL leads to a non correlated view of flight training, as it all just happens too fast. 4-5 weeks is more reasonable from a learning point of view.
Given the two I would strongly recommend the 8 month PPL over the 21 days - you'll be a much better Private Pilot at the end of it - and that way you won't go over on your CPL or IR - which I'm prepared to bet you will with a 21 day PPL.