I can get off thameslink low level and walk to the northern line with ease right?
With ease, but not that quickly! It's a hike.
If you're on Thameslink anyway, surely quicker to stay on and change to the Northern at Kentish Town (it's the right branch of the Northern there)? If you're at St P otherwise, then presumably easiest to trot across to the Northern Line at KX and do it in one go on the tube, given that the Kentish Town interchange isn't
that short.
I'm not clear how the earlier reference to the Victoria Line is relevant! (Oh, I see, is it one of those daft ideas from Google, that scourge of the internet...)
On the Northern - once you're on the platform at Kings Cross, I'd assume a journey time of up to 30 minutes to High Barnet, although it could be a bit longer if you have to get one heading to the other branch and change at Camden Town.