I genuinely don't care for any of them. All the candidates are equally bad. This often happens after a strong PM. After Thatcher there was dismal Major and then a clutch of lacklustre Conservative leaders, Hague, Howerd, the other one. After Blair (a strong leader whether you liked him or not) then Brown who was pretty poor, the Labour Party is left with a bunch of third-raters to lead in oppostion.
Labour's vote has declined since 1997 because that was an exceptional vote, a landslide. After the Conservative's 1982 landslide, they lost share at each ekection until they lost power. It is not a sign of terminal decline, just that the longer a party are in power, the more the electorate get fed up with them.