A hypothetical British B-21 purchase would require a much larger budget

Would the United States sell Britain the B-21?

The possibility is left open, but no sale is established and the expense would require a substantially larger British defence budget.

A British purchase is treated as a possibility, without establishing that the United States would approve a sale. Its substantial expense is the immediate constraint. A defence-budget increase of roughly a third is suggested as the scale of change that could make such spending conceivable.