Yes... all Stakeholder pensions wil allow you to do this. Whether you
*should* is another matter.
Get an IFA to obtain some figures for you to compare what you might get under a Stakeholder to what you are likely to get under S2P. You need to assess the "critical yield" - this is the investment return you need to achieve under the Stakeholder to match S2P. You then need to decide whether you can realistically expect to do better than that... if so, it's worth contracting out. (I personally doubt whether it would be worth contracting out on the current level of rebates though - they're not as generous as they used to be.)
See:
Probably irrelevant... you assess whether to contract out on a year by year basis.
Your NI rebate is increased by the basic rate tax relief. You don't get higher rate relief if you earn more than the higher rate limit because the rebate only relates to earnings subject to the basic rate anyway.
The tax relief is added to the rebate - it's all then invested together. The projected benefits are normally calculated on the basis of a real rate of return so the projected pension is in 'today's terms'.