If £8m/year is a fraction of a mid-size company cloud budget, I don't think you can class them as "mid-size"
I'm with a rather small company (~ 250 people) in the US and we pay about $1,2mm-1,4mm yearly on GCP alone.
Thus by definition that company wouldn't be mid-sized over here anyway.
edit: in-fact after checking even in the US, the IRS for example declares a large business as one with more than 10 million in assets, though there is no set rule like in the EU to be used by other gov orgs.