Like almost everywhere in Britain, many miles of hedges were taken out during the agricultural improvements following the Second World War. This largely stopped when the 1997 Hedgerow Regulations gave legal protection to most hedges. So nowadays there are four different problems.
One is neglect. About 36% are in poor or semi-derelict condition, as they gradually slump or get pulled apart by livestock and falling trees. The second is too-close flailing, which can prevent a dense shrub layer forming.
Thirdly, the use of agricultural fertilizers enriches the soils so much that rare and small flowers in the adjoining hedges get outcompeted by more aggressive nettles and cleavers. Finally, hedges in narrow lanes can get badly damaged at passing places and by the passage of large vehicles.