http://call18866.co.uk/index2.php - with an 0808 access number for non-BT use - not as convenient as prefix or CPS, but you still only pay for connected calls (unlike telestunt and telediscount, where you pay for the access call).
Also depends if you want to register and have a seperate bill, or just ring and go with telestunt and telediscount.
With 18866 *currently*, Uk calls are 0.5p/min, USA 1p/min and connection fee is only 1p - makes NTL's 3-2-1 tariff and 5p connection fee look sick, SICK,
SICK.