Depending on who you work for, an increasing number of employers are offering Cycle to Work schemes. Basically you choose an amount to spend on a bike which should be used for a portion (min 10%) of a commute to work, they deduct the value over 18 months from your gross salary, so you pay less NI and tax and so the actual net deduction can be around 45% off the bike price. We're about to buy something for £600 which will cost more like £350 from take home salary.
There are some catches, it can alter your final salary pension allowance if you retire soon, have to pay back if you leave the job etc etc, but worth investigating.
good luck
ben