I've ridden the Brampton Valley Way Market Harborough <> Northampton more times than I care to remember - it's a great ride (even when I got a p*ncture at 1am). I've never found lights of any help in the tunnels - just be aware of the dreadful surface, particularly under the vents (where you are likely to get wet!)
I firmly believe that there is a gap in the space-time continuum in Northampton, I've never successfully gone north to South without getting lost.
Thankfully, my trips were pretty much all to the northern edge of the place, with only odd forays on to Milton Keynes.
Enjoy your ride!