My two cents worth of information: (more like 5 bucks)
Most ebike motors and kits are made in china. Unless you want to spend lots of money on, example: Heinzmann german-made hub motor, or another similar motor .
imported Gas bicycle engine kits are the same way here in usa. The price on the kit is 200 dollars, but the "per piece, direct from factory" cost on alibaba is only 45 dollars each.
Build your own ebike. I wouldn't recommend purchasing on from a dedicated ebike seller that advertises though mass marketing, or has their own brick-and-mortar store unless its a well recommended universal kit thats custom made in the same country.
I see a lot of people pay big money for a low-end bionx ebike setup from a "certified" bike store, which isnt performance oriented. its like having a middle man. why pay for someone to stock and install the product for you? You know they are raising prices to make a profit.
Im not in UK, but I would avoid the "tesco" brand kit. Looks like a rebadged chinese kit. Doesnt mention any wattage (Ill assume its only a 200 watt kit) and the ad calls the lithium battery "state of the art", however lithium battery technology has been around for a very very long time already. Sounds like marketing to me.
Along with john lewis bikes. Why pay close to 2000 dollars for something that costs only 500 dollars in "cheng shin manufacturing co" parts?
If you want cheap kits, perhaps check ebay, (ebay.co.uk has 1000 watt pre-built wheel kits from what Isee at my computer screen)
Those seem to perform ok, and the parts are cheap.
Or you can purchase a quality, well reviewed and high-end motor, use a spoke calculator to purchase the right spoke length and build your own in-hub motor wheel according to the instructions on atomiczombie.com.