I don't have first-hand knowledge, but looking at the valves they sell
https://brimaruk.com/valves/brimar-tube-range/ the KT66, for example, appears to be 'pinch' construction and very closely resembles the ones made in China's Shuguang plant. So I assume that's where they come from. The ECC804 though says it "is made right here in the UK"
https://brimaruk.com/valves/triodes/brimar-ecc804/.
Cheers,
GJ