If the NowTV 2 month cinema offer for £5 is not shown on your Google Home App casting device under "Offers" then it looks to me that for some unknown reason you haven't got the NowTV offer available to redeem on your Chromecast.
I would double check that your Google Home App is up to date (my Android Phone is showing Google Home App version v188.8.131.52).
I found a way around this as previously mentioned
When you go the 3 step GET OFFER / existing customer page, copy the voucher code (beginning NOW---------) which is in your address bar, then go to the MY ACCOUNT menu at the top right and select MY PASSES. Then select apply a voucher and enter the copied voucher code from the previous page.
This worked for me . goodluck
First, I can confirm that your Google Home version number is exactly the same as mine on my android tablet.
Second, when I entered Google Home on my tablet the £5.00 offer was displayed, but when I pressed "redeem" my Google Home app stopped working. I tried two or three times and each times my app refused to work and redeem the offer. I have sent off a note to Google.
I will try again later after switching on Spotify, guess I'm gonna have to chill out again.
BTW, the Authentication error is given me problems again.
Oh, the Humanity.
You would not believed what's just happened, I was just chillin' out listening to Spotify, via Chromecast, when I decided to give this "2 month for £5.00" cinema pass another go.
So I got my tablet, which was already casting, opened Google Home, opened the offer page and tapped "redeem". Instead of just stopping, as it did before, my tablet asked me if I wanted to open the upcoming page in Chrome, I tapped yes, and there it was in the URL, the voucher number.
Quickly recovering from my shock, I carefully typed the code into my notepad, opened my Now TV account and copy and pasted the code into "My passes", I half expected the code not to work but it did.
So I now have two months worth of Now TV cinema, from now until January 2018, for the family and I to look forward to.
Guys, thank you for your instructions, help and encouragement, I would not have taken that final successful step without it.