If the deductions for the bike are taken from your gross (before tax) pay, Halfords are the legal owners of the bike from the start of the hire period, the bike is leased to you. Once you reach the end of your initial hire period, Halfords Cycle2Work will get in touch to discuss your options, one of which is Halfords' Managed Extended Hire.
If the deductions for the bike are taken from your net (after tax) pay you are the legal owner of the bike from the start of the hire period because you are not making any tax and NI saving while repaying it.