If Britain already had an operational anti-ballistic-missile system, retaining the capability through maintenance and replacement would have value. A system unable to stop a mass strike could still deal with individual weapons or small attacks. Removing those smaller threats would offer some protection despite the limits against larger salvos. An attacker would have to account for that defensive threshold.
Notable Point · 2:42:33-2:43:03
Why a limited ABM system can still matter
An operational anti-ballistic-missile system can offer protection against small attacks even when a mass strike would overwhelm it.