Isn't there a slight potential problem if the OP moves from "unemployed" status to "self-employed"? Will he then be ineligible for paying NI at the voluntary, unemployed rate, but have to pay according to his income? (a lot more, I think). Even though he does not have to declare an income for tax purposes (does he?), he may have to declare an income for NI purposes. There are (at least) 2 aspects to NI as I understand it: 1) It is partly a contribution for pension, sickness, unemployment, and possibly other benefits 2) It is partly a tax on income - for most employed people, the latter is the most significant part of it.
For contributions records purposes, these days you only need a 30-year contribution record to qualify for a full state pension (not sure about the other benefits).
But for the "income tax" aspects of NI, you have to go on paying NI so long as you are earning an income (above a certain threshold). I am not sure though whether it is only on taxable income (e.g. like a normal salary) though; if so then spreadbetting income may not apply, so I don't know how you/they would determine how much your NI contribution should be. It is, however, as others have said, important to keep up your contribution record if you ever hope to see a state pension (derisory though it may be), or other benefits.
UPDATE: Having spoken to someone who used to work for the Revenue, and knows a bit about NIC, apparently, self-employed people pay class 4 NIC, which is (in effect) purely an income tax as it carries no benefit value whatsoever.
I am not sure if it is possible to pay an additional contribution to secure benefits. This would be something to investigate if going down the "self-employed" route.
....googling brings up this. Seems like you pay Class 2 as well, to get your benefits, and may or may not have to pay class 4, depending on your profits:
http://www.businesslink.gov.uk/bdotg/action/layer?topicId=1073867961
http://www.businesslink.gov.uk/bdotg/action/layer?topicId=1073867963