>This being the case, just how could the hardware "be locked by the Pentagon at any time"?.
I think AC is saying that it's locked all the time, and only unlocked for test firings.
Probably the biggest issue with a broken system is that we might find out, in which case
the US's strongest ally and a useful second vote in the security council becomes rather hostile.
An independent British missile system actually strengthens theirs as well, an attacker can't assume that an attack on the US that stops the US threat won't be responded to by a British system.
A useless British system would compromise any strategy toward an enemy that presumably knows that it's useless, since AC's posting in the register know this then it's safe to say the rest of the world does as well.
Clearly the US could sneak in code that prevents UK Trident from working, the question I suppose is have they? Given the total destruction of their international relations it would cause, an awkward legal case when Britain tries to get a refund, and the subsequent doubt cast on their own deterrent it's probably more in their interests to hand over a working system.
Yes, they could shaft it with GPS, but it'd still get close enough.
BTW The Vanguard class carry 16 missiles each of which can have up to 8 warheads. In practice there won't be 8 warheads on a missile, since the MIRV footprint is only about the size of the UK limiting the number of different targets one can hit with one missile, besides you might not want to use 8 warheads at once so a spread of loadings will be on the boats.
I still reckon it should be dumped, we could get an airborne capability in a couple of years and with the new euro-cruise coming along that could carry warheads for comfortable distance, enough to put off anyone who cares from trying.