You'd have to go some, I averaged 16-17 moving speed on KAW - wouldn't leave much room for cafe stops!
I made 106km in 6 hours, riding from home to avebury off road (along the ridgeway, 47mm tyres in 650 rims, gravel bike), so similar speed, which is under 12 hours moving time for a 200km, leaving 2 hours for stops against the 14 hour time limit for BR
KAW is 350km, so split into two chunks of 175km with 25km on road added means it is challenging but not impossible. But if conditions are too muddy or there are mechanicals then it goes out of the window. Still it wouldn't be audacious if it was certain.