Well, if the BBC doesn't offer some kind of subscription service for Brits overseas...
My previous license payments and taxes will have already paid for BBC content produced before I left the UK
The fact you think you have the right to archive material merely because you have at some point paid for a TV licence is laughable. That's like saying you have a right to cheaper beer because you've drunk beer, p**ed it out and the resulting liquid is now beer again. You can't just magic up material from archives. It has to be researched, picked, possibly transferred to a playable format, maybe repaired, delivered, transferred to a server, played out to a device on which you can watch. Do you realise the manpower that involves? That's without studying contracts to work out who needs to be paid and how much. And paying them. And whether or not you hold the rights to distribute it in another part of the world or on the internet.
The only part of the BBC any taxes you may have paid in the UK will have paid for is when the government has contributed towards the World Service (so arguably propoganda) which you can access virtually anywhere in the world. World Service radio has some great content. Try it out. You might like it.
The BBC doesn't provide a subscription service for Brits overseas (and why should it?). It does however, offer a subscription service to anyone willing to pay in certain territories. Britbox isn't available in New Zealand, but some BBC content is available on Sky. The reason it doesn't offer content everywhere? Licensing. Television programmes and formats are sold by territory. Buying the rights to screen a television programme in one country does not give a broadcaster the right to broadcast it to another territory, as another broadcaster has paid good money for the rights.
So there you go. If you miss the BBC that much, maybe you should return to the UK and buy a TV licence.