Interesting debate, and one that has been revisited recently by the BMA. There are arguments both for and against, but in my opinion, no, its not ethically OK for the NHS to pay people for organ donations, principally because if the NHS were to pay for the organs, then it would be yet another drain on already stretched NHS resources.
There are wider considerations; It is likely for instance, that the donors would be those who are impoverished.... therefore the quality of the organs in question is debateable. It would devalue the human body, attaching a financial worth to various organs. It would likely bring in a "two-tier" system of health care, which is contrary to the spirit of the NHS.
You may find this article of interest;
http://www.studentbmj.com/issues/05/10/educati on/369.php
and this one;
http://www.bts.org.uk/approvedstatements.htm