well there are some questions you need to consider before choosing the best way
1. what and how is the bandwidth/latency/stability on your VPN?
2. how frequently you want the data to be copied across the network?
3. the best time slot for sending and receiving the backup on the VPN
4. other way to fulfill your needs
For the first question, bandwidth is the main concern on sending such a large file. 150Gb on a 10Mb/s DIA is like more than 4 hours of non-interrupted connection. you have to make sure your bandwidth is stable enough to be online and also with a acceptable latency to keep the connection alive.
Secondly, a file this big may take a very long time to be sent and received. so sending the file takes too long may affect the backup schedule some time when the network loading is too heavy during the send/receive.
thirdly, the free time foryour network i.e. non office hour at sydney, may not be the best time for UK to receive the file and receiving such a file will consume a lot of network and hardware resources. (storage needs to handle read and write intensively when receiving/sending the file)
last, how about an offsite backup locally? or scheduled backup with courier to send the backup media to UK? example: take a half month backup and sent to UK whilst a daily/weekly backup to stored locally at other local offfice or even Bank/Storage company.
this is what you can do with your exisitng environment.
of cause you can always upgrade your DIA/VPN bandwidth for faster transmission rate but its not a good practice to send such a large size file across the network frequently.
