Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)

If the OP does not plan to stop over en route to or from IST, then he should be charged about the same fuel surcharge regardless of whether he flies BA or IB. The only thing he would avoid by routing with IB is the UK passenger service charge for connecting through London. (Of course, there might be some airport fees for connecting through MAD, but they are likely to be lower than the UK PSC.)

A test booking at the ITA website should show fuel surcharges and airport fees for both routings. Whether the OP can find award seats is, of course, another matter.