Rob....Yes it would be easy to set up a bike frame manufacture in the U.K...but very expensive to build a competitive one.I like the description Rose use of themselves: "Quality bike assembly in Germany."
Investing in a ground-up plant to make aluminium frame ebikes in the UK cannot make any sense.
However, there are still quite a few steel frame builders who could easily make a dedicated ebike frame.
Equipping a frame building workshop from scratch wouldn't be expensive.
Components drawn from the usual suspects, assembly in-house, and you would have a unique ebike brand which could fairly be described as British.
The frame manufacturer in Tianjin,extrudes his tube from the raw billet....that is an expensive extrude machine.He bends the tube on CNC benders.The welding is robotic. The paint shop is largely automatic. The stickers are affixed with very low cost labour.....you would need Chinese volumes to consider the investment in the U.K. !!!
KudosDave