Fix link to OS's updated licening page

When I looked up the licensing terms on this site for using latlng data derived from postcodes, I came across a broken link on http://www.ordnancesurvey.co.uk/

I _think_ this the corrected page, but I'm not as close to the source material as you were, so please let me know this PR links to the right one.
Add the Northern Ireland bit back in

Wow, I totally missed the second licence. That'll teach me to edit in the browser, with funny text wrap…
Merge pull request #36 from mrchrisadams/patch-1

Fix link to OS's updated licencing page