Just read the smallprint at the bottom of the page:
Disclaimer: The buyer of this item takes all responsibility for his/her own actions when riding any bike fitted with an engine, we will not be held responsible for any injury caused by riding the bicycle or the engine itself.
This mountain bike engine is not suitable for anyone under the age of 18 years, the buyer takes ALL responsabilities when purchasing the item.
Customers in the UK will require the bicycle to be registered as a scooter if the buyer wishes to ride the bicycle on a public highway, you will also need to have the bike inspected by VOSA and have it taxed and insured if used on a public highway.
Private use when not on a public highway is acceptable and does not require the above VOSA checks and registration.
So on a normal scooter it's £15 tax and £55 regristration fees. I don't know how much inspecton is... But doesn't that mean you will have to have a numberplate lmao! On a cycle haha
Either way it's alot cheaper than a moped - Just try and stay off the road, use the woods to get places

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She's still alive! -23/02/2017