Once the payments are made, though, don't you either hand the bike back, or pay off the reamianing balance? It has taken my employer a long time to get their heads around the scheme, and they've really tried to plan it out to avoid abuse/misuse by employees, largely to avoid a sudden rush of people wishing to join in November, the bosses main concern being people abusing the scheme to purchase bikes for their offspring. They've limited the period that you can sign up in to a 4 week period in late spring. I think this is a reasonable decision, if C2W is to encourage people who would not usually consider C2W then they at least have some weeks (or maybe months) of less that shoot weather to get into the habit.
My understanding of the scheme, and I'm hoping to use my employers scheme in the spring, is that you can purchase a bike of up to £1000 (or bike & accessories up to the same value). You get vouchers to use at the appropriate bike shop, and then make part payments over what I understood to be a fixed 12 month period. At the end of 12 months your (now second hand) bike has a fixed market value, and if you wish to keep it you pay the final market value, or hand it back. So theoretically if you make all your payments in 1 month, wouldn't your fixed market value be significantly higher than it would be 12 months down the line? This would mean less of a saving than the 12 month payment scheme.
On a salary sacrifice scheme the payments are taken pretax, so you pay proportionatly less tax & NI (pension as well if in an employer scheme). I'm unsure if taking one month to make your payemnts would be in your interests, as spreading them out for longer would mean less tax & NI contributions for you & your employer.
Can we be cheeky and ask why you're keen to make all your payments in one month, and importantly will your monthly salary be enough to cover 12 months worth of payments?