Hi there,
You need to do a few things.
1) Speak to someone to get free debt advice:
www.citizensadvice.org.uk
www.nationaldebtline.co.uk
www.cccs.co.uk
www.payplan.com
2) Write to each of your credit card issuers asking for a copy of your credit agreement under the Consumer Credit Agreement (CCA) 1974 s78.
You will need to send them £1 for this. (as you have already done)
If you have already received copies of the agreements create a thread for these and get some help reviewing them for enforceability. (Scan in images) If the agreements are April 2007 onwards they are enforceable.
Not paying the regular payments will cause them to escalate their pursuit of you.
You should tell them that the debt is in dispute until you have received a compliant Consumer Credit Agreement and that you will only deal with them in writing, as this is harrasment.
Going down the route of the CCA to be declared unenforceable does not write off the debt, it just means that they cannot get it enforced by the court.
As you have found out they can still pester the hell out of you, but stating that they can only contact you by letter should reduce this.
You can also ask them to remove your phone number from their records.
3) Search the forums for the posts already on how to deal with the various debt collection agencies (DCAs) and what tactics they use, and what their letters really mean. (lots of bluff and thunder).
4) Share your experiences to get some advice and support
HTH
Sparky