A british flag and a AR-15-like rifle? Does some british units use those?

Perhaps she had the choice between the L85A1 and the AR-based rifle here.

I know what I'd choose

She could be a special forces soldier using a Colt Canada C8 more likely I think she could be a PMC because that camo is not British.

Or more than likely the artist just thought British soldiers were cool but since American weapons are more popular he merged the two.

SAS iirc use the Colt Canada C8s.

Wanna draw cool British people and L85s suck? Most of your realistic options are going to be AR derivatives anyway! :D

Her plate carrier is also USMC issue.