We started with battery powered ANR sets also by sennheiser. They were very good but its not very enviromentally friendly to be disposing of all those AA bats every month, and they would always run out just after take off on a "long" flight. So when they decided to issue headsets to each pilot (mainly for health reasons) , A/C powered ones were the way to go and very nice they are too.
The mod was done by DHL I don't know how expensive it was but lets face it it's aviation so it probably wasn't cheap. I'm not sure what the fit was with BA.
Hope that helps.