Another update that I hope is helpful!
After making several inquiries I decided to go for the aforementioned tow bar (bike hitch) for the bike carrier. But fitted by an independent fitter. It cost £600 and is the exact same one Honda fit. Because the electrics only connect to the car lights it did not require dedicated electrics and "should" not impact on the warranty.
There is, however, another issue that may put others off. The towbar manufacturer says that the bumper cut is invisible. They got this wrong. The cut is visible. It's not huge but clearly noticable. In fairness, the supplier at their cost offered to remove the towbar and pay for a new bumper. After a little thought, we decided it was ok.
I will put photos up in a day or two. I may wait until the new bike rack arrives to show it all.