The worst in the past in terms of dangerous policies might perhaps be Tories 1983 vs Labour 2005.
Anyway, both parties are at the moment hopelessly divided and uncontrollable, or at least uncontrolled, by their leaders. Until recently, I used to say that the First Law of British Politics was that any party would sooner or later be dragged hopelessly to the right by its leader (Thatcher/Blair/Clegg). Nowadays, both parties are led by people on the left of their party (slightly so in the case of Cameron, very much so in the case of Corbyn), except that 'led' is a bit of a misnomer in both cases, and the parties are imploding through infighting over policies, and even more through the personal ambitions of the individuals hoping to be the next leader.
Part of it is that IMO neither Party Leader really likes being a political leader. Cameron likes strutting around and looking important but the actual WORK of a leader is something else, and indeed he's already indicated that he will be quitting. His constant tendency to leave the actual work to other people is now exploding in his face as they have turned to fighting each other. Corbyn almost certainly never expected to be leader, but simply to show that the Left still exists and to promote some key Left policies. His lack of interest in power makes him more likeable as a person - but not effective as a leader of a party which tends to be divided and fractious at its best.
Actually there is nothing intrinsically wrong with within-party division; it is better than everyone being robots or cloned sheep. If 'wets' in the Thatcher era, or anti-war Labourites in the Blair era, had been able to have more influence, it would have been a very good thing! The problem is that our system is not really set up to manage such divisions. In some Europaean countries, virtually all governments consist of multi-party coalitions, and with some exceptions, they find ways of handling and negotiating the divisions. In the USA pre-Reagan, and to some extent beyond, it was automatically accepted that there would be liberals and right-wingers in both parties, though the proportions differed between the parties. This is still to some extent true of the Democrats (though contrary to some DU myths, there were FAR more truly right-wing Democrats 40 years ago than now). In our system, there is very little room for MPs voting independently on most issues, and thus there seems to be nothing much in between a strong leader 'sitting' on the party and pushing their own agenda and stifling dissent, and a weak leader watching helplessly as the party imitates the Cats of Kilkenny ('...so they fought and they fit and they scratched and they bit , till instead of two cats there weren't any!')