Your current Base + 2% is indeed just like a tracker rate. There are some tracker rates in the market at better than this, but not without massive arrangement fees that you would never recoup because the savings on the monhtly payments wouldn't be that big.
So. If you want a variable rate, stick with what you have. Of course, Base rate is going to rise this year and next, of that there is little doubt. If you would rather have the comfort of knowing what your monthly payments would be, then you should get a fixed rate. Waiting until rates start to increase before taking out a fixed is too late, because future increases will already have been priced into the fixed rates on offer at that time.
Depending on your circumstances, Woolwich has a 2-year fixed at 3.79% at the moment which is among the best available, but bearing in mind that Base would need to go up by 1.29% for your existing mortgage to get that high, you would likely be better off on a tracker if you want the best deal in the long term. I would be looking to keep hold of your Nationwide BMR deal until around November 2011, then grabbing a fixed rate for perhaps a 5-year term which you should be able to get for around 4.75% or so at that point. That would likely represent best value for money in the long term.