All Season Green Vegetables?

[mindatrisk]

Hello!

I am a complete newbie and I know almost zilch about anything gardening, and so I need your help, please

I volunteer with drug addicts and alcoholics, and as part of a program to provide them with greater nutrition to support their recovery we are going to provide 'green juices'. These will be made with organic kale, spinach, celery, cucumber, coriander primarily.

To make this cost effective we need to be growing our own green vegetables all year around. Given that I am in the north east of England and the winter can be very harsh, which green vegetables can we grow throughout the whole year, and which varieties in particular?

Thank you!