It still works. Just log in to ancestry.co.uk. Click on the Home tab along the top of the screen.Then scroll down - it's about midway down the page. The offer expires on 29th May.
Yes it depends what you want - 4 months was enough for me at the moment, but a warning that these special offers aren't always so special.
I had to get rid of all the Ancestry cookies to get it to work.
The 12 months Premium Membership link worked today£59.99https://www.ancestry.co.uk/cs/offers/subscribe?dna=crossAct