Unfortunately, there is no 'cheapest' way. Any CPL/IR is going to cost at least £40k. A realistic estimate is probably another £10k on top. If you factor in a type rating as well, add £30k more. There are some scholarships out there (google HCAP). But not for the full whack I believe, unless you're female (google Easyjet scholarship).
However, it can be done...many have worked/begged/borrowed the funds.
A word of warning; in my experience, graduates from CTC/Oxford seem to be far more employable by a lot of UK airlines....