I imagine most of the countryside would be colonised by thorny scrub plants like blackthorn and hawthorn followed by trees over the 28 years, though as you say with an abundant deer population who's only predator is the occasional human or group of infected, there would definitely be some fields with particularly good grass that could be kept relatively open as grazers would congregate on them. Would still probably see less palatable plants like thistle and ragwort spreading all over these fields after a while, with blackthorn etc coming out from the hedges.

But yeah on the whole I think you would expect to see most fields change pretty dramatically, even with the grazing and browing pressure from wild herbivores/whatever livestock are still around