Blight can be prevented by spraying with Bordeaux Mixture - a post which said Bordeaux mixture was to be withdrawn, on this page, was MISTAKEN - your website compiler was reading info on the RHS website wrong. There have been a number of attempts to withdraw Bordeaux mixture over the years but it is still currently available. If you spray with Bordeaux mixture you will need to re-apply it after rain!
On tomatoes, pick off any affected leaves and burn or bin them (don't compost diseased material), spraying with bordeaux mixture to stop it spreading. Tomatoes in greenhouses, protected from the rain, are less vulnerable.
If your potatoes have already formed a decent crop - and I don't know about you, but my 'first earlies' are the size of maincrop, while my 'second earlies' rival some of the smaller moons of Jupiter - then you can simply cut off all the foliage and bin it. Thus will stop the blight migrating down to the spuds and you can lift them in your own time.
If there is any suggestion that blight might be about, store your potatoes in hessian sacks rather than paper. If you don't have a nice dark cellar (how many of us have one of those?) Then use the sacks double to keep out the light.