Fly Ryanair to Trieste in Italy or to Pula in Croatia (both 1 hour 15 minutes drive). Ryanair flies daily to Trieste from London Stansted (2 hour flight time) and 3 days a week to Pula. Croatia Airlines flies direct Heathrow to Pula. Slightly further away but still within reach are Treviso (Ryanair) and Venice Marco Polo (British Airways). Another option is to fly EasyJet to Ljubljana in Slovenia (3hrs 30mins). This route opens up the option of a day or two in the beautiful Slovenian lakes of Bled and Bohinj.
If you are coming from Europe you can simply arrive with the car.