I've just returned from a 2-week visit to the UK (Devon), driving both ways. I used the 2 variants on the "Serbia" route for comparison purposes, not having done this trip for 4 years now. The following observations may be of interest to people contemplating the drive over -
Sofia to Devon:
For the trip over, I did Serbia/Croatia/Slovenia/Austria/Germany/France to Boulogne.
Left Sofia at 6.30 on a Sunday morning. Slow going to the border at Kalotina because of extensive road-works for the 1st half of the 60kms.
Crossed into Serbia around 6.30 CET. Discovered no exchange bureaus open till 7am so couldn't buy any dinari (which was only in case of being stopped by Serbian police, who can issue instant fines but only accept Serbian money). Decided to hell with it.
There are 2 diversions before reaching Nis, where the motorway starts. The first is about 30kms from the border and is 7-8kms of dirt track. The 2nd one is 25kms long, winding thru hills, and brings you out at Nis, avoiding the 13 tunnels section. It is notionally asphalt but large sections are under repair, so presumably the diversion is going to be in place for a while. Fortunately HGVs are prohibited on it.
This 100kms from the Bulgarian border to Nis is the slowest part of the entire trip. Took me over 2hrs.
Once on the motorway its plain sailing pretty much all the way to the English Channel. Minimal delays at Serbian/Croatian and Croatin/Slovenian (EU) borders. By the way, didn't see one traffic cop in Serbia (mind, it was early Sunday morning). Croatian motorway now completed all the way to Serbian border. And about 75% of the road across Slovenia is now motorway, with the balance probably not too far away.
Motorway tolls: Serbia: 15 euro Nis/Belgrade and 12 euros Belgrade/Croatia. Croatia: one toll of 15 euro. No less than 4 toll points crossing Slovenia - paid by credit card and it totalled about 22 euro. Austria: one toll of 9.50 euro. France (entry near Strasbourg, then A4/A26/A16 to Boulogne): total of 44.40 euro. But absolutely superb motorways. Oh, and the Karawanken tunnel between Slovenia and Austria - 6.50 euro. Total in tolls - 120.40 euro.
I took 2 days for the trip. On day one, got as far as Germany, 60kms or so inside Austrian border, and stayed the night at a internet-booked small hotel. 1,215kms from Sofia and it had taken me about 15 hours. Day 2 was another 1,090kms to Boulogne, so 2,305kms from Sofia to the channel. That leg took me just on 12 hrs (my jeep is only good for 130-140kph).
Arrived Boulogne in plenty of time for 8.25pm crossing of Speed Ferries, pre-booked at 43 euro. First time I've used it and was very impressed with loading/sailing/unloading times. Probably only 20 or so mins longer than Eurotunnel.
Then I had another 390kms to get down to Exmouth in Devon. That took 4 hrs (using M3/A303) plus a meal break. Total time on the road on day 2 was around 20hrs!
Devon to Sofia:
For the return trip I decided on 2 changes - avoiding France/tolls and using Hungary rather than Croatia/Slovenia.
For this reason, I decided to use the channel tunnel to save the extra 30kms from Boulogne to Calais. Internet pricing from Eurotunnel for one-way was about 220 euro, but I got a price of 104 euro from Direct Ferries website. I ran with that. Got there early and was able to go on an earlier train without extra charge. The channel tunnel has to be the easiest way to cross - pity they charge like wounded bulls (and still losing money).
Then it was up the French coast to Dunkirk and then east on the E42 all the way to Cologne (Koln), picking up the A3 autobahn for the nearly 700kms to Austria. Stopped for the night in the town of Passau just inside the border. About 1,050kms from the channel.
On day 2, I crossed Austria - on this route (past Vienna) you have to buy a vignette - and then into Hungary, another vignette. About 20kms before Budapest, take the ring-road from the M1 to the M5 motorway. There was a huge, huge tail-back on the ring-road - at least 15kms - but fortunately going the other way. The M5 is now motorway all the way to Szeged and the Serbian border and marvellous it is to drive on.
The stretch from the Hungarian border to Belgrade is fairly slow going. The first part is single (tho wide) lane and 100kph speed limit. Then there is an extensive patch of motorway preparation, much of which is 60kph. But I didn't see a single traffic cop again in Serbia. Connected with the motorway east at Belgrade around 6pm, rush-hour, so was slow going getting across the Sava river and out of the city.
Stopped for some dinner at the OMV on the motorway to Nis then the final push to Sofia. Travelling eastwards, there's the same 25km diversion from just east of Nis - I was thankful for my trusty Nissan Patrol and thinking it wld not be a good place to break down late at night, when I reached it. But the other diversion is only for westbound traffic. Finally rolled into Sofia 1am CET, 17 hrs after leaving Passau.
Tolls this way - Austria: vignette for 7.60 euro, Hungary: vignette for 6 euro and Serbia: 2 of 6 euro from Hungarian border to Belgrade and 15 euro, Belgrade to Nis. So total of 40.60 euro, 80 euro less than the westbound trip.
Soo, for anyone looking to make the drive either way (and who's read this far!), the only challenge with Serbia is the 100km section from where the motorway finishes at Nis to the Bulgarian border. Plus fairly expensive toll charges. Toll-wise its cheaper to go the Hungary/Serbia route than the Slovenia/Croatia/Serbia route (33 euro versus 66.50 euro). But the distances from the channel to Sofia were almost identical both routes, at around 2,300km.
For anyone just wanting to get there in as short a time as possible, either of these routes are vastly preferable to avoiding Serbia and going via Romania and the Danube crossing. Having got to Szeged, I couldn't have psyched myself up to then having to do the 400km down Romania to the Danube (no motorway, a zillion villages), then facing the indeterminate time to cross the Danube, then having another 250km to Sofia.
Ok that's it. Hope its helpful to somebody.