The best way to deal with the IR valuation is at the end of the year (assuming your employer gives you the chance to buy the bike) is to to ask your employer to fill in a P11d and treat the bike as a taxable benefit. So instead of paying an additional 25%, you pay tax on the residual value, on a £1000 bike that would be 20% of £250 = £50 (assuming you're not a higher rate taxpayer). This will be collected through an adjustment of your PAYE code so you don't actually have to part with the cash.
This only works if your employer actually takes ownership of the bike at the end of the year, some schemes like Cyclscheme have it written into the contract that the bike passes to them unless the employer opts out in which case that wouldn't work.
The other option is do what I've done, which is to not bother buying the bike but continue to use it. This only works if you trust your employer and you're intending to stay for a decent spell. Edited to add, again this only works if your employer owns the bike.