Why are flights from Tripoli to Tunis such a ridiculous price?

going by car to Djerba can be dangerous and personally i wouldn't go past Janzour or Tajoura.

maybe you've heard of the rule of supply and demand ?
a lot of people want to leave the country ( high demand ) and there are only a few airlines connecting with Tripoli ( low supply ).

therefore 1+1= 2 ( expensive tickets )

problem solved.

Not really. I should have pointed out that n th internet there are no direct flights from Tripoli to Djerba. They all require two or three changes. Flying to Istanbul first. There are direct flights toTunisia but not are listed on the internet.

If you're prepared to travel to Musrata I believe there are currently flights from there to Istanbul via Turkish airlines (there may be a layover before flying on to the UK)
As for flights from the UK, at the moment it is possible to book flights from many UK airports for January flights to Istanbul with a possible overnight stay in Turkey, onto Musrata later on or the next day.
For example I checked last week and the cost for a return flight in January from Birmingham airport UK (BHX) to Musrata via Turkish airlines was £450, operating daily. I would imagine it is a similar cost from London or Manchester. If this is an option for you you can book directly with Turkish airlines, although they told me that unless the layover was 24 hours or more they dont bear the cost. Alternatively it is possible to book via a travel agent called Arab Tours of London (easily found on google) who quoted me £460 and said Turkish Airlines pay for the layover.
Have no idea if this helps, but good luck with it all :-)