There wouldn't be much point broadcasting a 720p US outlet in 1080/25p (or 29.97p) in the UK unless that network was originating 1080i/p internally and cross-converting to 720p for TX (which is what lots of European 720p broadcasters do but I think fewer US networks do.)
So what do BT Sport ESPN do with the various "HD" feeds they get from local sports channels in the US for their baseball and NBA coverage?
I suspect it all arrives 1080i as harshy has said.
Some events are covered in 1080i even if the local rights-holder broadcaster is 720p - as other clients will want 1080i too...
Another interesting anomaly is that NRK and SVT are both 720p for broadcast - but internally they are almost entirely 1080i (they cross convert on TX...). ARD and ZDF were similar (though they accepted programmes made in both standards) - though now they have 1080p outlets on DVB-T2 they are presumably running 1080i or 1080p (or higher) internally.
Ideally from this point on, HD origination should be 1080p (often referred to as '3G', but nothing to do with mobile phone networks !)
UHD/4k formats are all progressive, so you really don't want to be upscaling native 1080i stuff, and of course
1080p to 720p conversion doesn't require any temporal messing about.