Lets ease the worry first. Debt collectors cannot enter your house, they cannot take any of your posessions or list or levy them. Bailiffs are the only ones who can do this, and bailiffs cannot get involved unless ordered by a court after a county court judgement.
(Even bailiffs cannot enter your house unless you invite them in. They can enter via an open window or door in which case they would then have legal powers to enter again).
Never speak to them on the phone if they should find your number and call you and tell you parents the same. Stick to your original 'only in writing' stance.
If you should get caught by a debt collector, you can tell him to leave your property and call the police if he refuses.
Write and ask for a signed copy of the credit agreement enclosing a £1 cheque and stating that the cheque is for the copy agreement only and not to be used for any other reason.
I say a cheque because if you use a postal order, it should be crossed and if they send it back, you will have a job cancelling it and gettin your £1 + fee back. Don't send a cheque from your own account, ask your parents if they will give you one.
What ever correspondence you send them, do not sign it with your usual signature, alter it in some definable way.
You will find a letter in here, you want letter N1.
Creditors and DCAs - Letter Templates & Budget Planner