Companies either use a 3rd party who administer the scheme or do it themselves. The advantage of using a 3rd party is that it makes it easy to set up, and disadvantages are that you'll have less control, they'll take a cut, and may restrict your choice of bike shop. The arguments obviously switch round if you set up your own scheme. From the employees' perspective a company-administered scheme is probably preferable (assuming they maximise the benefits for the employee). Although it's all good really as it will mean a higher-rate taxpayer will get a bike for around half-price
Setting up your own scheme is actually pretty simple. There is info on the IR site. It's recommended that you agree the details of the scheme with your particular IR office in advance just to avoid any awkward questions later. The value of the loan also needs to be taken account, and if I remember rightly, any amount over £1,000 will require companies to set up credit arrangements. For that reason companies have tended to cap schemes at £1,000. We used to offer £1,5000 but reduced it to £1,000 when the credit rule was introduced.