If you give yourself a break and then regulate your work rate on the big ride I think you'll be fine.
As you state, I have been using Zero tablets and they are really good, I totally agree with you. I use FGS post ride on all long rides, and usually have one in the car when driving home post Swinley. On my rides I have learnt that my best performance comes from a slow start in the first 5 miles, thereafter the biggest problem is now 'reining myself in' (I could never say that before,
Food wise, I like some or all of; mini-bread rolls with scrambled egg, Waitrose mini-flap jacks (they come in a tub and are worth trying) and bananas - these have all worked and are proven for myself. Sacrilege I know, but cake mid ride does not help me - a bacon sarnie... that is a different matter.. yum, yum.
A big advantage on the Ridgeway ride will be that my riding partner has done many long rides on the road over the years and I know I can trust his judgement on pace.