I've been to a couple and they are interesting. I enjoyed it too. My first one was Uxbridge and I knew the money behind the venture comes from Tesco, but the brains behind the idea are the Tollys who also own the Taylor St. Baristas stores in London, and that Tesco's only had a minority share in the company. I was very pleasantly surprised. Was expecting a step up from the normal chains, but found something that was very close to a lot of the good independent cafe's in London.
Union coffee provides their beans, and I've had mixed results from their beans in the past, but something has changed I think and they are getting substantially better.
An 8oz flat white is pretty large. They generally hover around 5-6oz. Temperature wise, a flat white should be ready to drink as soon as you get it.