I'd be interested to know how them asking for your contacts' email addresses sits with data protection legislation.

a) Is it legal for me to give their contact details away without their permission (does the legislation apply to private individuals or not)?

b) (Depending on jurisdiction) they're not allowed to hold, use, disseminate or otherwise do anything with those email addresses without the permission of the account owners, which they don't have. So on what legal basis are they asking for it and can I sue them?

(Huffington Post UK has a registered address of 

Shropshire House
11-20 Capper Street
London
WC1E 6JA
United Kingdom

which would presumably give UK courts jurisdiction over their UK site?)