I think, Ladbrokes, that they would bolt next spring because of going through the winter, most onions being biennial and flowering in their second season.
The Japanese varieties used in autumn planting, and newer ones such as Radr and Electric, are somehow bred to resist that, as long as they are not sown (seed) before about 23rd August, or planted (sets) before about late September, so they are good to plant now.
This year I had lovely early, white onions from leaving some overwntered White Lisbon spring onions to bulb up in May and June. You could still (just) sow them now but indoors only, for planting early March under fleece. Really they wanted sowing late August.