as your riding on the road you'll need 'E' marked tyres to be legal.
There isn't a massive choice of knobbly but road legal tyres available. The most popular choice is Slashers, these are widely available and are cheap but they are also only 4 ply and I used to pick punctures up all the time when I had them. They also don't last long on the back of the street legal Raptor 700.
Another choice is Maxxis, either the Razr 1 or Razr 2. These are avialble E marked but not all are so you'll have to check before you buy. They are great tyres for laning and they last twice as long as Slashers but they are twice the price.