There are many possible routes you can take, it all depends on how or what you want, if you wish to go the quickest then take the main route (depending on where you cross from UK) into Germany, Austria, Hungary, Romania into BG, i've done the red eye in 37 hours tho not recommended. I now take upto 2 weeks to get to Burgas (i also stay for the Summer so no rush), afterall you are passing some of Europe's finest Cities it would be a pity to miss them. I take a different(ish) route every year stopping off at some fantastic places, the Czech Republic is awesome, the drive from Dresden to Decin in the Czech along the Elbe to name just one. If you need any pointers, just ask.