In Ireland (and I believe it's similar in GB), beef cattle are usually finished on some mix of silage and concentrated feed with a significant maize component to promote better fat distribution.

There's a continuum between 'extensive' and 'intensive' finishing methods - the former takes longer and uses more forage & grass, and is best suited to native breeds. The latter uses more silage & concentrate, and is used for 'continental' breeds.

Dairy cows will also have pelleted additives over the winter, making up to about a quarter of their intake (largely depending on silage quality). But those tend to be mixes of yeast, fats, and digestible fibre so shouldn't necessarily require imports.