All very good advice, which ever camp you are in about shortest / most direct!
I echo response re always wear life jacket - no point having it if not worn on every flight over water! (OK, maybe not Biggin to Southend over the Thames!)
If you always avoid the long trips over water, and especially a climb over water things like leaving Jersey to fly direct to Southampton is never going to happen.
The other option for a trip into France then on to Spain is to break it up into decent legs and you may find a route via Folkestone to Cap Gris Nez then on to Tours, then down to Perpignon, then on into Spain. Long legs but quite feasible in a PA-28. But this was from Biggin, so from the midlands you may want to route west of London then due south but then longer leg over water. But then you could aim for Rennes, and then on south. Beziers did not have customs so we went to Perpginon, then direct Palma on one trip, another we did Sabadell (near Barcelona) then onto Ibiza. to get to the Balarics you are also looking at trips over water, and there are not as many ships in the med as the channel!
Overall; if you are planning on doing this sort of proper touring, which is great fun and quite easy, then the distances and legs involved are each of a few hours each anyway, so a 20 minute flight over water is probably the least of your worries. Being prepared mentally if the worst happens is of course vital,.
Have fun, enjoy, and report back!