Why not escrow the ￡400m the UK owes Iran?

I was down in Bovington recently, at the Army Tank Museum - a great day out btw, highly recommend it - when we came upon the tank in the picture above. It looks a lot like an MoD Chieftan tank, in fact it is a Chieftan tank, albeit modified and renamed the Khalid and originally bound for export to Iran. The information board reads, "The Khalid is effectively an improved Chieftan, notable in terms of the engine which is a Perkins Condor V12 diesel. It was designed by Royal Ordnance for the Iranian Government under the name Shir 1 in 1974 but five years later, following the Revolution, the order was cancelled. However that same year the Royal Jordanian Army ordered a modified version of Shir 1 to be named Khalid."

So what you see is the genesis of the whole UK-Iran-Nazanin Zaghari-Ratcliffe saga. A tank deal that was worth ￡400m, the money was paid but the tanks not delivered. Nobody disputes that the UK owes Iran the money, but thanks to sanctions the UK cannot hand over the funds. Because of this any dual nationality citizen going to Iran stands to be kidnapped by the authorities - trumped up charges do not cover for what is essentially a state backed kidnap - as Nazanin, Anoosheh Ashoori and Morad Tahbaz have found out to their cost.

I am no international lawyer but is there not some sort of UN escrow account into which HM Treasury can deposit the funds? The international community freezes funds all the time, think Syria, Myanmar, Zimbabwe, Venezuela etc. In going down this route would it not be up to the Iranians to show the world, not just the UK, that it has changed its ways and is worthy of the funds?

Failing that why not send Iran ￡400m worth of Covid jabs, they certainly need them.

I'd be interested to hear the whys and the wherefores from those who might know.