I've just changed from Halo with complete Wifi to Fibre 1 and BT want the Wifi discs back, Do I understand right, that if I don't return them, I get charged £30 each for them ? So is it OK to keep them and pay ??
That is correct that if you do not return them you will be charged £30 per disc.
If you do want to keep them you should contact BT 0800.800.150 and inform them of that so that you are not plagued with reminders.