If they're nearly full size (well - they won't get a lot bigger now), as Leggi says, this normally happens. Leave them for a few weeks to ripen in the hot sunshine (you can now let the weeds grow round them - it helps) then harvest when the foliage has died down.
Most onions at our allotment site have got downy mildew on them this year
I feel pretty sure Yorkshiremans problem is the long dry spell. Onions are shallow rooting and have suffered this year. My large onion bed have all wilted in quite a different way to the natural autumn gradual ripening we are used to. They look in a sad state.
Mine have flopped I thought it was them nearly ready for lifting it seems funny that they have done it just after a few days when it has rained well so the aren't dry.
Mine which have been well watered, luckily, are looking great. Some top growth has flopped, but they are growing dramatically. One or two are small and getting their skin, but the rest are doing ok.
Got a picture finally!
Nothing wrong with those, top growth has just become a bit heavy for them, some I planted early look exactly the same, the bulbs will still be growing and the weather has been so hot they may be ripening early. I really wouldn't worry.