You cannot put a BA and AB reward on the same ticket or it triggers long haul business class Air Passenger Duty which adds £130 or so to the tax.

AB is fine, I did NYC to Berlin twice last year, once with the new fully flat seat. Its not CW but its hugely cheaper. Remember that the return is overnight so you'll be asleep anyway!

If going from London, I would second the city flight out purely for the experience, as long as it isn't too far out of the way. Do the one that does immigration in Shannon.