From working in the terminal in retail/customer service, I see countless American/Canadian travellers everyday. Most having flown in to LHR/LGW and flying to Europe or coming from European hubs and flying out of STN on the vast network of routes it offers to Europe and beyond.
I don’t have figure to support this but having seen this with my own eyes it clear to me that flights to the US are needed and should work.
In the summer holidays I remember speaking to a Canadian family who had come to London for a holiday. Having travelled to Buffalo from Canada got JetBlue to MCO and TCX from their to STN as it was cheaper, than any other option at the time.
Pick holes in this post if you want but I don’t think it’s fair to pick holes in Primera until they have started operating and can prove to be doing well or not doing well.