I'm not totally up to date with the pass rates of Cranfield and BGS, but people who have done their exams via BGS swear by them and many have passed with 90%+ pass rates first time. But I've not heared bad reports from CATS either.
If you go with a cheaper school it may be false economy? i.e. You'll have to pay for the re-sits and take time off work/drive to the test center etc.
You may end up spending the same money but with 2nd time passes
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And as portsharbourflyer said, when you're looking at spending £40k / £50k on training the £1k is neither here no there.
Have you looked into the cost of a Multi-IR Test? it's a hard test and some need to re-sit it
. I think it's wiser to invest in a good school none the less - "Buy cheap buy twice"
(this isn't aimed at the ground schools above - more at training schools and pretty much anything in life)
Like FTO's / Modular v Integrated / BGS v CATS ... go with what suits you.