Wow, I used to live just 500 yards down the road from where you'll be staying. Don't know anything about the B&B though.
It should only be about a 15 min walk to & from the B&B to the nearest U-Tube stop, Finsbury Park. But since you're an American, I'm assuming, and that you guys don't walk anywhere, I'll translate that for you. It'll be a 5 min taxi ride to that Tube stop.