Endemol hold the worldwide broadcasting rights to Peaky Blinders. They've sold the UK streaming rights to BT but they'll be hoping to sell similar rights to different companies across the world. (Netflix have already bought the streaming rights for Japan). The system only works (and brings back enough money for the production companies, Tiger and the BBC, to continue making quality drama) if each streaming service ensures that it complies with its contractual conditions to only make the programmes available in those territories where it has bought the rights to do so.
So BT MUST try to block access to its streaming services outside of the UK.
As an analogy, anyone with a big enough satellite dish in Italy can receive signals from the Astra satellites used by Sky but - because they only hold the UK broadcasting rights for their programming - Sky is barred from selling subscriptions to their services to anyone outside of this country. Similarly, the BBC must block overseas access to most of the content on BBC iPlayer.