I don't have much to add to some of the good comments above, other than to add that it's not just illness or starvation that will cause the body to break down muscle, but also inactivity. When I had my knee immobilised in a brace for 6 weeks I ended up with one normal leg and one joke stick leg. Took months of hard boring rehab to get it back.
As others have said, if you're exercising normally it just isn't going to happen. My guess (and it's only a guess) is that you'd be so knackered from glycogen depletion that you'd grind to a halt before you start recycling muscle in an ordinary day ride. Things may be different on a multi day ultra effort.
As to all things in cycling the key is fun. Make your rides enjoyable and interesting. Experiment with different kinds of food, different pacing etc. to find what works for you and ... have fun!
I can sympathise with this. But just a little "cafe" combination lock could be enough for you to get in and out of a cafe/shop to buy some water if you are in a position to always have a half an eye on the bike. Scope the place out first before entering. Can you see the till from outside? Alternatively there are sometimes taps in churchyards that people use for flowers that you can get water from. I carry quite a lot of gear with me so a couple of locks don't make much difference. But if you're riding light it could be a problem. I normally have one ordinary and one oversize bottle for rides in the range 100-150km (60-90 miles). You could also start out with a 330ml or 500ml bottle in a jersey pocket.
You say you ride in the Surrey Hills. Here are some cafes that have good sight lines:
- National Trust Cafe at the top of Box Hill. Served from a hatch. Just take your bike with you in the queue. Yes, there's often a queue. Racks are provided that are 100% visible. And if you don't want coffee/cake there's a drinking fountain where you can fill your bottle.
- Destination Bike in Box Hill Village. Has racks outside that are visible from inside.
https://destinationbike.co.uk/the-café
- Heartwork Coffee. Holmbury St Mary This is all pretty much open air. Keep your bike with you.
https://www.heartworkcoffee.co.uk/