Standard base coat uses a solvent thinner. In some range it is similar to the 2K thinner. I recently bought Lechler BSB for my modernish Jaguar from Paints4u.com. It sprays well with a good match once the clear went over the top.
Water base goes on nicely but for you, there is no advantage and so I would recommend solvent basecoat. If your locals are jobsworths, then get it mail order from the likes of Jawell or Express paint.
100% agree - there are also disadvantages in water based that will make your life harder. If its cold i.e. anytime now in the North! you need IR to make the stuff dry sometime in your lifetime.. all the time its wet it will be subject to dust and if you doing this at home in less than perfect conditions that's a nuisance.
There is one minor advantage, if you make a mess of it, if your a DIY sprayer who might not be happy at first pass you can just wash it off and do it again.