The homebase/B&Q/Atlantic Homecare would all sell blinds for standard window sizes fairly cheaply. Not sure about the 'trim to size' aspect - I'd imagine these would never really work out to be as neat as 'made to measure'. We used 'click for blinds' on the net - polish company - cheaper than getting made to measure here, and good customer service.
No affiliation with them - just satisified customer.