Let me add a discussion starter of my own too.
Reading past threads, the Boardman Hybrid donor bike of choice seems to be the Hybrid Comp version at around £500 new.
Aside: Boardmans have 4 ranges of bike, from bottom to top Race, Comp, Team and Pro, running from about £400-450 to about £1000-£1250, with more and better branded components and quality as you go up the range.
I'd suggest that that is a bit of a false economy, because by stepping up to the next level - Team - you get a lot of mid-range branded components (eg Mavic XM319 wheels, which are a nice solid mid-range MTB wheel, and hydraulic disk brakes) which are arguably all much better suited to the extra stresses imposed by a conversion - especially if you ride it far and quickly. Replacement costs should be pushed that much further in the future.
You also get a carbon front fork which is more comfortable, but I'd welcome opinions on that as an E-Bike component.
The move up to the Team level will cost £750 not £500, but there are two possibilites:
1 - 18-19% discount is regulation at Halfords. 10% for one of their seemingly monthly discount campaigns, and 10% more for membership of British Cycling which gets you insurance too. That makes your £750 more like £600.
2 - These bikes are a Cycle to Work favourite, and are often available on ebay for under £500 either because a replacement is purchased soon afterwards, or it was reassessed as too difficult.
Just my thoughts.
Ferdinand