Why Britain did not build deliberately inferior warships

Did Britain build weak export warships on the assumption that it might later have to fight them?

No; Britain generally relied on superiority in numbers and the better training that a larger fleet could sustain.

Britain did not normally design intentionally weak ships for export on the assumption that it might fight them later. Policy instead relied on having superior numbers available. Those numbers also supported more extensive training. Fleet size provided both a quantitative and qualitative advantage.