>Actually I don't think we'd have even used it then
Absolutely we would have. In practice its existence is sufficient to force the embassy to push out individuals wanted for serious offences as 'or else' has actual teeth.
>So it's very much the nuclear option. If we ever use it, then we've massively reduced the protection
The Police used the Act the year after it was passed to break into the Cambodian Embassy and serve eviction notices.