Bolting Cauli!

Hi
I tried cauliflower (All Year Round) again this year, still no success. I have attached photos of one, all others the same, and grown in different parts of allotment. The seeds were started off in my plastic greenhouse on 25 March, then planted out when big enough on 5 May. They seem to grow initially 'leggy' , as if they don't have quite enough day light - not too high, so I repot them lower down to the first leaf height. Even when transplanted to the allotment the stems grow quite tall, then when the head starts to form it bolts after being about ping pong to tennis ball size. The plot was manured with chicken pellets and the plants were firmly planted.
Any ideas what I can do differently next season? Should I try overwinter variety to harvest May/June. If so, any suggestions on variety to use. Also any advice on how to get half decent caulis much appreciated. Thanks.

Funny that I have been able to grow Clapton and Aarlsmere with no trouble but AYR made heads a bit like lots of white broccoli stalks crushed together lengthways.

Whatever it is they needed, they didn't get it here either