You can now upgrade for free to Business First Class on Heathrow Express trains when visiting London, if you are a Star Alliance Gold status holder (such as with Air New Zealand). Enjoy more legroom, complimentary magazines and newspapers - even if it's only for 15 minutes. When I rode in Business First Class a few years ago, I made sure to take as many free magazines as I could to read in my hotel, and on the plane on the way home.
To receive this offer you must purchase of a full fare standard class ticket to and/or from London Paddington and Heathrow Airport T2, and show your valid Star Alliance Gold Frequent Flyer card when prompted in Business First Class on the train.
This promotion started on the 15 February 2016 and does not have an end date. See here for the full terms and conditions.