A show has to be wildly popular before Americans even get the chance to be exposed to it, which really limits the amount of viewers for UK programing, but that's no reason to assume that shows have to be recast and refilmed before an appreciable number of Americans will be interested in them.
If American producers want to remake shows from the UK I wish they'd do it with panel shows. It's an entertaining genre seen in many countries that just never took off in the US. At least then they'd be more justified in bringing in American talent so guests are more recognizable.