As far as manufacturing in the US, I will admit I do not know if the FDA would have an issue with it being in Sawston. It will be logistically tricky, they will have to get the patients leuko from some hospital in the US to Sawston in 24 hours (IIUC). But the FDA does not regulate that.
According to somebody in the field who posts over on IV there are regulations on international transport of cells that play in, but I have no idea what he is talking about. But if true, could make the 24 hours tricky.
Why can't they cryopreserve the tumor using NWBOs patented method via Healthbank (NWBOish subsidiary), ship to Sawston, and then do leukapheresis. I would bet my socks (clean) that this shipping validation has been done. I would also believe the international shipping of tumor is not a biggie but maybe trickie initially till Customs and their mama get the routine down.