I think the legal distance you are allowed to veer off the right of way is 5m either side. But as Jimbob says the trespass caused by avoiding obstruction is a civil matter and the cause of the obstruction may over ride the nuisance of an enforced trespass. It is a criminal offence to obstruct or attempt tp stop someone using a right of way, I personally think that not enough effort is made by local authorities to make landowners and property owners realise that bad husbandry which causes obstruction is an obstruction.
Because landowners are choosing to become involved in agricultural practice, to lessen their liability in capital transfer tax, they tend to disregard the responsibilities of husbandry they imposed on their tenants in tenancy agreements, when it was more convenient for them just to gather rents and be free of the harder work involved in farming. It has always struck me that local authorities are remiss in not billing landowners for money spent in clearing obstruction, such as seasonal overgrowth and worse as it was once considered good husbandry to maintain hedges, cut out verges and repair drainage so that the old ways did not deteriorate.
A hedge is as much a crop as as those seasonally planted, it will decline in usefulness if allowed to overgrow, it will become less stock proof is the bottom is choked with weeds, it would be a good idea if local authorities collected some old tenancy agreements, read them and started to instruct landowners to follow the example their forefathers expected of their tenants.