I did this with a pal earlier this year, ’twas good!
I too did it on 25mm tyres (Planet X gutter snipes), the Berwick bit was a touch sketchy but I quite enjoyed it, my pal less so.
There’s another muddy bit later on but I can’t quite remember where exactly, the weaving in and out of coast heading between Berwick and Newcastle is a bit demoralising and I think a more inland route would be better as it would be more direct. There was some steep little climbs up and around a cottage or two then back down hill to only have to repeat it again a few miles down the road.
We bivvied on the beach at bamburgh after about 125miles, 5 pints of Guinness, 2 single malts and a steak & ale pie in the local in bamburgh.
2nd day was a scoosh, awesome tail wind, glorious sun (made up for the previous wet day), we booked a suite at the Marriott in Newcastle so spent time in spa when we got to Newcastle.
Enjoy.