HRM seems to show bad pace data - can I get data solely from my watch?

I've just run London Landmarks Half Marathon, with my Garmin Fenix 7S connected to my Garmin HRM Pro Plus. My watch/HRM showed I ran 24.8km at a pace of 5:00/km (a 1:44 half marathon) however the official time for the half was 2:04 - which would give a pace of 5:53min/km.

I ran another timed half marathon 2 weeks ago in 1:58 (5:36 min/km), so I know what my pacing feels like - I felt I ran a lot quicker today so I'm a bit disappointed but then my Garmin doesn't look right either. 

Is there a way to download pace/distance data solely from the watch? I think maybe the HRM might have throw the GPS off. 

Any thoughts?