Well, I got some new informations:
1. Whole package (kit) for converting to Panda Van costs about 50€ and there are a couple of kits ready.
2. I've dialed some numbers to find out the procedure on how to export unregistered vehicles from Serbia and I got dizzy and also got some stomach troubles 'cause of idiotic bureaucracy so I can advise you to stay away from that idea.
On the other hand it is quite simple to export already registered vehicle, all you need is proof of sales (in Netherlands this proof/contract is used as legal paper at their's DVLA, don't know about GB) and permission to drive the car given by owner, all sorted in about an hour...