After the first three months, you'll need to pay £8.99 per month to keep Stadia Pro. You can let the subscription expire, though, and still have access to Stadia Base, a basic storefront that lets you buy games individually and play them (via streaming) in up to 1080p resolution.
BT's offer is available until January 30th. Then, on February 7th, the company will offer various "Superfast Stadia gaming plans" that include a Stadia Premiere Edition. It's not clear exactly how these deals will differ from what's being offered today. (BT has merely confirmed that they'll be available with Superfast Fibre 2, Ultrafast Fibre 100 and Ultrafast Fibre 250.) An ongoing Stadia Pro discount, for instance, would be intriguing.
For Google, it's an obvious move. Stadia's launch was met with some lukewarm press coverage -- a curious platform with promise, many concluded, but not something that everyone needs just yet. For now, Google's biggest problem is awareness. It can't increase Stadia's install base unless people know it exists. The company made some attention-grabbing TV adverts, but it's unclear if they've made a real dent in the public consciousness. Recruiting BT, which often takes out massive billboard ads, will only help its cause in the UK. Still, it could be difficult to penetrate a market that is already rife with gaming platforms and subscriptions.