If there are low numbers of voters within a constituency for a particular party, and none of them has the inclination or ability to become the "local" MP then it is bound to be the case that a non-local might stand as a candidate.
In the case of the Lib Dems, I suspect they are matching the candidates of both Labour and Conservative parties in number, in part to prove a point when the count of votes shows they should have a similar number of sitting MPs but the "first past the post" system doesn't allow it to happen.
As for your local situation, AvadaKedavra, has it been a "safe seat" for years, and was there a Lib Dem candidate in 2005 (can check on the BBC Election 2010 website pages). In either case I'd expect the Lib Dems to put up a candidate, partly for the votes count, and partly to see if people who are fed up of "punch and judy" constantly bickering will vote for an alternative, even if you find the prospect of it being someone from 'afar' a little disconcerting.
(Especially given the fact they are likely to be working with staff and/or volunteers in a local office, who will be able to fill in gaps in knowledge about local issues, and will be dealing with lots of the day-to-day issues while an MP is down in London.)