Sorry to here you are having a nightmare
It sounds like the head gasket has gone again, but really the only way to find out is to take it to bits again.
If it turns out they have messed up they are liable for the repairs but be carefull they will try to wiggle out of it.
There are tests you can do buy checking the coolant for fuel and oil then you can narrow it down and pressure testing the system, and compression leak down tests, and block leak checks they need to work out if it the head or block.
It can be tricky to machine a warp out of a aluminium cylinder head. if you clamp it wrong you can take some of the warp out before you have even machined it. so you have to be very careful , you can also make it worse as well.
The other problem is the cam shaft runs in the head so if the head is warped and machined flat the cam will not run properly with in it's bearing faces in extreme cases.
There is only a little difference in the jcw head to the standard head. from what I know its a light bit of porting work to inlet and exhaust nothing major.
If you need a second hand head get a standard one, you can get the porting done or not depending on the money BMW Mini Cooper S Cylinder Head Complete, with Delivery Mileage only | eBay http://www.ebay.co.uk/itm/Mini-Coope...8#ht_500wt_922
I would speak to citizens advice they will be able to tell you about your rights and give you advice.
It may also be worth trying to get hold of some one like tony franks or another mini engine specialist and ask if it is possible for the block to give these symptoms, it may not be possible due to the design but I don't know, in which case it would aim you at the head and gasket.
Sorry I cant be more helpfully it's hard to say with out seeing it. keep us up to date and I am shore someone here at mini2 will be able to help on way or another.