Several UK dealers will order a CBA for you, so long as they are official distributors for the make you are interested in. Most of them deal with one or two Italian manufacturers, and they should be able to offer you access to catalogues of the available models. Emilio Allodi used to deal with Fantini, but Im not sure if he still has any business with them. Best ask him if you need to.
You pick the colour and the button configurations and thats about it. You are usually required to leave a deposit with the dealer, and all being well you should have your accordion in a few months. You may get a surprise at the price of a brand new accordion, and dont forget the UK dealer needs a mark up to make it worth his/her while.
Nothing beats try before you buy, which obviously wouldnt be an option with an order placed via a dealer. There is a caveat with ordering unseen and unplayed as well. If for any reason the deal falls through, or the instrument turns out not to be to your liking, you are in a bit of a dilemma. If you attempt to recover the cost of any money you have paid, beware that if you are having the instrument specially constructed to your own specification, the Bank or credit card company is entitled to claim that the accordion was custom built, and as such will not pay out. Basically, you couldnt source the instrument in the UK, so were obliged to place a special one off custom order. Remember they will be trying to sell the unwanted instrument to minimise their loss, and custom built is a term they love to hit you with in the event of any claim. The time required to construct an accordion is also against you, as most claims have to be submitted within 90 days of placing the order. Youll probably know that legal advice in anybodys country isnt cheap.
If you are happy to run the risks involved then by all means use that method, but there have been one or two horror stories on here about similar situations. A lot of people have ordered by precisely the same means with no trouble at all, but I am one of lifes unlucky buyers, and my experience was enough for me never to trust any third party to order a musical instrument on my behalf. If its not in the shop, Im not interested.
Personally, Id try and source a half decent used box in Brittany. Youll get to see, hear, and play it first, and most French retailers offer a minimum guarantee of one year. Might give you an excuse to re-visit Brittany if something needs fixing.
Renting is something which is possible in France, and may even be possible in the UK, although I doubt whether many people in the UK would have a CBA for rent.
As Donn says, the main issue with the accordion is what you hear from behind isnt the same as what an audience will pick up. If you record yourselves playing you should get the drift.
If you get the chance in Brittany, try and listen to an accordionist accompanying one of the local mouth blown wind instruments. If you can hear the accordionist at all over the bombarde then youll be doing very well indeed. How the players know what is coming out of the front of the instrument is beyond me.
The accordionist gets a break every now and then when the bombarde player runs out of puff. Bad enough when its just a duet, but in a full blown band with flutes, fiddles, drums, and all sorts of other loud gadgets, then the accordionist would be as well just taking the reeds out and miming.
Maybe the above clip wasnt a very good example of what I meant.
When it all gets going in the Auvergne the accordionist is little more than a stage decoration. Bass and chords might get through occasionally, but you could play any melody you wanted and nobody would know the difference:-
Devotees of the music will tell you they can hear the accordions, but they know what to listen for. To me it just all gets lost in the mix. Left Stage - 6 Right Stage (accordionists) - 1. The one point is for the time hes beating with his sabots. The lady with the diatonic appears to be playing Silent Night.
I know Ill probably get stick for those comments from lovers of French folk, and perhaps I have exaggerated the situation. Occasionally the accordionists get to play a solo or two, but hopefully the second clip will serve to illustrate the plight of the accordion in such a line up. Accordions can be pretty loud, but up against a bagpipe and a hurdy gurdy, unless you have the constitution of Desperate Dan (a UK comic character of immense physical strength), then you might as well save the back ache and get the paper and comb out.