A southern English weekend can be booked in sunshine and pitched in rain. The forecast is allowed to change. The trip still works if you decided in advance which parts are optional.
The night before
Look at the rain and the wind, not just the little sun icon. Wind is what makes a tent miserable. If a gale is coming, pick the more sheltered pitch and put the door on the lee side. If it is only showers, you mostly need a dry place to cook and a mat that keeps you off the wet ground. Tell yourself which walk you will drop. Deciding that at 7am in a damp porch is harder than deciding it at the kitchen table.
On the field
Pitch taut, guy lines out, and nothing touching the inner if you can help it. Put the bag you need for the night in the tent before the rain starts, not after. Cook early. A hot meal in a gap between showers is worth more than a plan to walk six miles and eat late. If the field floods in one corner, move. Pride is a poor groundsheet.
The next morning
A wet pack-up is normal. Shake what you can, bag the wet tent separately from the sleeping bag, and hang it up the day you get home. The route you skipped is still there next month. Getting home with a tent you can use again is the successful version of a changed forecast.