Unfortunately the legalities of parking on the pavement are not as clear as they could or should be, outside London at least. Yellow lines do apply to the verge and the pavement, but it is the decision of local authorities over how and where they enforce this as it very much depends on the local environment in terms of pavement width, availability of parking, housing density and so on.

Rule 244 of the Highway Code draws an important distinction between London and the rest of the UK, as it states: "You MUST NOT park partially or wholly on the pavement in London, and should not do so elsewhere unless signs permit it." The key factor here is the use of 'should not' as opposed to 'must not', which creates the grey area for interpretation on a local level. You can appeal the fine based on the fact that the disabled space was not available and that your wife is disabled, but it will be in the opinion of the local authority as to whether this is sufficient grounds to uphold the appeal.