According to the DFT website,passengers are expected to present their carry-on liquids in a "single,transparent,re-sealable plastic bag(about the size of a small freezer bag) which itself must not exceed 1 litre in capacity(approx 20cm x 20cm)".
So in my opinion security are out of order if they are insisting that you buy the bags from one of their vending machines as long as you conform to the DFT requirements.
Sounds like another way of Manchester Airport trying to make money out of the public.