I don't know the specific deal, and it does seem a little too good to be true. The reality is that it almost certainly will cost you more because virtually nobody ever does a licence in minimum hours. Also check whether this includes exams, test, aeroplane for test...
But if the people seem happy, try it and see if it works.
Personally I would recommend doing JAR PPL so long as you can pass the medical - the syllabus and test standards are broadly the same, and it gives you more options later (night, IMC, flying overseas, microlights without needing an extra skill test...).
Regarding paying up front versus pay-as-you-fly; ALWAYS go for the latter; this is an unstable business and far too many people have lost money with apparently good value "pay up front" schemes. That they refused to take your money up front I would regard as a strong point in their favour.
Regarding Billins and Bonus, I know both well. Both have good instructors, okay aeroplanes, and you'll learn to fly from a busy airport (with regularly grumpy air traffic controllers) after which nowhere will ever scare you! Billins is more attuned to PPL training, Bonus to commercial training. Availability of instructors seems better at Billins, but there are more instructors and aeroplanes at Bonus, both suffer slightly from the vagaries of Cranfield - but that's not their fault. Billins does come across as a bit amateurish around the edges, but I'd really not let that worry you; the actual flying is very professional, which is what matters.
G