Yes you can. Check the seed packet for when they are supposed to mature, and there is time for sowing through May, sorry about the slugs, they are a plague this year.
I sow all my winter brassicas (except spring cauli & swedes), at the end of April because in East Yorkshire later sowings can result in insufficient growth before winter. I prefer to sow in a well prepared, sheltered seed bed on the allotment but last year moles & flea beetles caused problems. I`ve hedged my bets this year by sowing some in the ground and some in compost in 70mm x 120mm plastic cups. I never look at the dates on the packets.
Sheltered seedbeds are the way to go! I had really good germination and growing in one of them filled with compost. I think i will pregrow brasicass in future in this way.