Just looked in the Tuckers seed catalogue - says you can plant up to the end of April. Some varieties are for overwintering (got Jermor in myself), but Sante should be planted mid-April - says in a difficult season, planting earlier may result in bolting; others sown mid Feb to end of April.
If a heavy frost is forecast, then use protection - fleece or something...
Boxing Day is supposed to be the traditional day for sowing Onions (perhaps this is for show as Kelsae, the big thing, should be sown Dec-Jan)