BritBox has more than 500,000 subscribers

"BritBox, the US SVoD service from BBC Studios, the commercial arm of the BBC, and ITV, the UK’s biggest commercial broadcaster, has surpassed 500,000 subscribers, doubling year-on-year, as it ends its 22nd month in the US and 11th month in Canada.
The service, home to a library collection of British streaming content, was considered the fastest growing subscription on-demand streaming service in the US from Q4 2017 through to Q3 2018"
Source:  Advanced Television, 18th January 2019