Thanks for the ride +Sir_Shannonball +MikeMC and I rode it on Friday. We went via the Lea Navigation direct to High Beech and did the complete loop from there. A really lovely route and it was (just about) fine on what were actually 27mm tyres (Open Pave). I'm glad it was dry and I'm sure my aching muscles would have appreciated something wider. Anyway, I'd certainly recommend the route and it might be even better if modified to cut out some of the Fairlop bit around the official start.
And not only did we find lots of bluebells, we even had the deer crossing our path + some lovely views back into London.
Thanks. The closest I have to a gravel bike is my Lynskey Sportive Disc; possibly a slightly earlier model than yours. Its supposed to take 32mm without mudguards but when I tried a few years ago I couldn't fit 32mm at the rear. I even measured the tyre and it was bang on. I can fit a 32mm at the front, possibly bigger. In any case, I'm too lazy to change tyres for one ride so I'll see how I do with 28mm.