Broccoli or Calabrese? question

In the spring I bought what I thought was broccoli plants and stuck them in an area where my first early potatoes had failed (I think blackleg got them) They've since been doing really well and early this week I noticed they've been forming small crowns, now I was under the impression these would not appear until the winter. I've lost the name of the variety and have never grown broccoli before so I don't know whether I should leave them to grow into bigger crowns? I'm concerned that they are bolting due to the dry weather as the crowns are a little brown in colour.  I did a quick search on here and wonder if they might be Calabrese as I think they mature in the same season. Can anyone shed light on this and advise me to the best way to treat them to encourage a good crop and prevent the flowers from opening?

There are broccoli (aka calabrese) plants that will be ready for harvest at most seasons of the year. So make sure you read the harvest notes on the packets/labels

If you have flower buds now, you can't stop the flowers opening, just pick them and eat them before they open, and wait for more small sideshoots to form, just as peapod said

Harvest for Marathon is mid July and August, so they are doing what they should.
You have to judge it right so that you pick the heads just before the flowers open.....whatever size they happens to be.

You will get more of these smaller heads once you have picked the first ones 

oh thanks, I'll pick them soon then! great