You are proposing driving about 1000 miles just to get to the Croatian border if you start from say the Chunnel and then have to come back again. That is probably 7 days or half the holiday at least just driving, estimating 300 miles a day which is more than enough per day even if there are no delays. Surely you won't want to be driving for more than 5 hours per day with breaks? Then you have overnight stops which if you planned well need not be overly expensive but depends how much comfort you require.
Certainly to drive there would be a really lovely trip but you wouldn't see much other than nice scenery for a lot of it. You really need much longer than 2 weeks to make it a really memorable trip. Seems pointless to drive through a country and not stop for several days at least. It is a potential nightmare at times to get from Warwickshire to Foklestone. It took me 9 hours last time from lancashire to Folkestone as a result of crashes, road works etc. so good job I had booked tunnel crossing for following morning. Dartford tunnel/bridge queues were ridiculous, parked up for ages.
With petrol, chunnel and tolls, overnight stops etc. etc. it will start to look very expensive and you will really leave yourself little time in Croatia itself. Add in cancellation of flights etc. and it really is a no brainer.
You could look at the Michelin route planner at http://www.viamichelin.co.uk/ which will give you all the relevant mileages, different routes and estimated costs and you could investigate stopping points and accomodation.
I think for a 2 week trip you are better using flights and hire a car for when you get to Croatia. Train might be a pleasant way of completing some of this lengthy journey. This will probably be the most cost effective and far less exhausting than spending all your time driving.
This is a trip I may do myself later this year in my Z4 but plan to do Spain first. Did some of France last year but didn't rush. Unfortunately had to come back early so didn't do as much as I would have liked. These sort of trips involve huge distances and are costly when all is added in. To make it sensible you really need an extended time and a number of stopping points where you can base for several days at least and do day visits in the region. Fortunately being retired I can take as much time as needed so time is not a constraint and self catering bases are the way to go for me as these are cheaper than hotels and you can come and go as you please for day trips.
Enjoy your holiday but leave the car until you have more time.
Forgot to add. Your insurance for EU would need checking as it would probably be extra often considerably so. Usually it is only for a limited time and only 3rd party. Very few companies give fully comprehensive for more than 30 days without additional cost. Again, being a retired wrinkly helps, as Saga give 1 year EU fully comprehensive cover in the standard policy which is amazing.
