Stop worrying.
If you can't pay a debt, just tell the lender exactly that. Offer to make small monthly payments. They can either accept or refuse. If they refuse, all that will happen (at worst) is that they'll seek a County Court Judgement against you. The courts accept that people can't pay what they haven't got, so the CCJ will simply order you to pay a small amount each month (which will probably be the same, or possibly even much less, than what you offered the lender in the first place).
'Court action' might sound very ominous but many people find that CCJs make their life much easier, because (as long as the small regular payments are kept up) the lender is effectively barred from doing anything more drastic (such as using the services of bailiffs).
If you're still worried, speak to the Consumer Credit Counselling Service. (They're an independent charity so, unlike many so-called 'debt helplines', they won't be trying to sell you anything. They just want to help):
http://www.cccs.co.uk/
Chris