This is London, it rains. The sun also shines on days when it’s not supposed to. On the day forecasts are good but not always helpful. Sometimes when the day starts overcast and drizzly, by 11am it’s clear. Some of the best walks with old maps have been on days when the weather has been a bit ify.
You may decide to cancel if you hear a forescast predicting rain. You might on the other hand decide to take a chance and make the walk an adventure. If it gets bad we simply take shelter or maybe stop for coffee.
The great thing about my walks with old maps compared to conventional guided walks is I am flexible and am able to adapt the walk.