There are broadly two types of legal ebikes in this price range.
Lighter, less powerful bikes such as the Orbea/
Ribble with the Ebikemotion hub motor, and heavier more powerful bikes, usually with the Bosch crank drive motor which most of the Cubes you've seen use.
Another light assist system is the Fazua crank drive, which is fitted to Boardmans and others.
Yet another possibility is the Halford's Carrera hub drive bike, which is similar in power to a Bosch bike, but much cheaper at around £1,000.
Sheffield is known to be hilly, which would point towards a Bosch/Carrera bike.
However, much will depend on your weight, how much grunt you can contribute, and how far you want to go.