I assume that if you are after carcass "quality" you are referring to quality grade, of which marbling is the primary component.
Not all British breeds are great at marbling and in general don't have as high of carcass quality grades as most other British breeds like Angus, Red Angus, Shorthorn, and South Devon. Now, that being said, just because an animal is an Angus doesn't mean that they automatically have good marbling. There are several lines of Angus cattle that don't marble all that great.
Other breeds and composites to consider if you are serious about carcass quality grades are South Devons, SimAngus, and Braunvieh. Braunviehs may be one of the best kept secrets in the cattle business (and I am not a Braunvieh breeder). They are one of the better marbling continental breeds in general, but like with other breeds, not all Braunviehs are high marbling so you need to pay attention to ultrasound data and EPDs.