unfortunately unless you have the £345 fee for the official receiver fees you can't. The £150 of the £495 full fee, which goes to the court, can be waived if on certain benefits but the £345 must be paid.
Depending on your debt level you may want to wait until April when the new Debt Relief Orders are due to be in place.
If you have no assets, debt maximum of £15K and no disposable income you could use this route instead. I believe the cost will be around £100 and basically achieves all that bankruptcy does.