In previous years it has generally been possible to do as you suggest provided you get the minimum tariff points.
This summer I am aware of specific instances where the situation you describe did not result in an accepted application - the grades in question were exactly as our caee - the candidate was denied a place for getting ABC when the uni had required BBB.
The reason is pretty easy to understand - unis project how many candidates will fail to get the grades requested and they 'over-offer' places on the basis that statistically a small percentage of applicants will fail to achieve the grades required. This summer, fewer candidates fell short as hence some unis were desperate to find reasons to decline a few candidates. They get penalised by the Government if they take in too many candidates.
You'd better bank on getting the exact grades required as a minimum.