The main reason LCY based crew won't operate the lcy-nyc route is because they don't have experience of the long haul operation and therefore no training in regards to the long haul product. I'm sure that with the appropriate training they will be more than capable of providing the same service that mainline does.
Also, their MOA would have to be completely re-written (at LGW we only need a little adjusment for this operation). I'm sure the LCY base hasn't got a vast amount of "extra" crew, as a fair amount of crew would have to be taken off line to do training. I'm not sure if they have the capacity for that at the moment.
In regards to safety training, they have excellent standards at LCY, the same as the rest of the company, whether mainline, subsidiary or franchise.
Gg