Rhagoletis alternata
As far as I can tell the wing pattern is unique to this species, although some other species come fairly close.
I'm not sure how common this species is (not many dots on the map for Norfolk at the NBN Atlas, but that doesn't necessarily mean much). I found this one indoors.
, North Elmham, 4-Jul-21 (A) (1).jpg)
, North Elmham, 4-Jul-21 (A) (2).jpg)
, North Elmham, 4-Jul-21 (A) (3).jpg)
, North Elmham, 4-Jul-21 (A) (4).jpg)
female Rhagoletis alternata showing aculeus and close-up of tip of aculeus, North Elmham (Norfolk, UK), 4th July 2021