There’s one golden rule you must learn. It concerns the good old British weather. The newspaper tipsters, and the selections on this site, are judged on the going the day before racing. So if there is a change in going, especially a drastic one (i.e. Good to Heavy). Then obviously these selections are false, and should be ignored. If there is a minor change (i.e. Good-Firm to Firm) then you should still check each individual selection to ensure that it acts on that going.
NEVER back a horse that has never run on ground totally different than it has ever raced on, as long-term you will be paying for the bookies holiday.
Although I never totally void a selection, I will reduce stakes depending how severe the weather change is, and how previous results have gone. Sometimes it may be poring down, but race times only change a little.