idk to what extent the claim is true, but off the top of my head Britain's GCHQ discovered public key cryptography but for some reason kept it under wraps and made no use of it after WWII. It was only found out years after it was rediscovered by American researchers.
If we're chucking in on these, which seems to be the case, also Black Arrow; the UK's nascent space programme which in 1971 put a satellite into orbit (which is small beans now but in 1971 was still a serious achievement) and was then cancelled because the US said they'd give the UK discount pricing on their own launches, and then when it was cancelled the US said "Ha! Suckers! Of course you're not getting a discount!" (accounts of this may vary)
I think the UK might be the only country that developed, and then fully abandoned, its own capability to launch satellites. Now being slowly recreated, fifty years later.
