I think there is far less crossover between LD / labour votes than commonly stated.
20 - 30 years ago, they were a party of the regions, gaining seats in places where the voters felt too far removed from London, so seats in places like south west England, rural Wales, the less populous parts of Scotland. As voter mobility increased, that side of the party has vanished (hence losing places like North Norfolk / the Cornish seats in recent years)
Now, they're the party of the internationalist wealthy upper middle class - a more distinct demographic, but one concentrated in places like university towns / south west London / the more upmarket suburbs of northern cities. Hence the focus on 'orange book' issues in recent elections rather than the Kennedy/Ashdown era focus.