You're paying for use of somebody else's property in installments but it is not your bike even when you complete all installments. Your employer will no doubt offer you the option to buy it from them, so look at the end options. HMRC got wise to significantly lower than full market value buy out agreements, you could find yourself paying out a chunk of money to take ownership of the bike or it remaining your employers for a year or more longer while it depreciates to a point where you can buy it for a peppercorn price.
Factor that in to your costs and how long it may be before you get to really own the bike.