I just think that it,s the time of year
I'm inclined to agree - especially the strawberries & peas if the weather has been cool and wet for a while then got a bit humid & warm, and they were unlikely to keep on going forever. Now would be a good time to clear the beds and remove any mouldy fruit and old leaves from strawberries to get more air circulating round the plants.
Will it reoccur next year?
most likely as the spores are everywhere and airborne at this time of year - mildew is like any other fungi and mushrooms that you see now, producing fruit bodies and releasing spores. Even in the wild, blackberries and other fruits will be succumbing to mildew as it is nature's way of composting
Chard is very hardy and I think should be alright. I'm not sure about lamb's lettuce (I don't grow it) but I think what you are seeing is most likely to be surface coating and the result of the weather (very damp & humid) I'm also wondering if your garden is not getting enough air movement (is it well sheltered, especially if surrounded by trees and bushes) and does it get very damp at this time of year? It sounds a lot like my greenhouses - I closed the doors the other week because the temperatures started to drop and I still had a lot of chillies & peppers to ripen but forgot to open them when the weather warmed up .... dare I mention that I have been kicking myself for not checking sooner
Re: courgettes and cucumbers - in the right conditions, they do get mildew readily and is not unusual. You could try growing varieties that are more resistant in future.