Nazanin Zaghari is a dual British-Iranian citizen who has been detained for years because of allegations over plotting to topple the Iranian government. These claims are clearly fictitious, and it's widely believed she's being held hostage until the UK government repays a supposed Iranian debt from many decades ago.
Aside from a few strong words, the UK government has, in reality, done very little to nothing so far, and at present seems to be exploring options to pay off said debt to secure her release. The UK has a reasonably strong military presence however, and other countries have shown they can interfere when willing (the US famously assassinated Qasem Soleimani for instance.)
Why wouldn't the UK government conduct a military rescue mission to recapture Nazanin Zaghari and free her from Iran?