I had a quick google and I'm not sure you can buy those motors in the UK. Mybe they're marketed under a different name?
I think a gravel bike with a 48v 328 rpm Q128C rear motor and a 52V battery would give you an acceptable cruising speed, and with a battery with enough charge to go full power all the way (one way), it would still be lighter than your Haibike. The kit with a 52v 17.5Ah battery would weigh about 8kg. Look for a donor with hydraulic discs, which will be much more reliable for long distance commuting. They're a lot more expensive, but worth it. especially when you only have thin tyres and need to have good modulation to prevent skidding.