As I understand it there are significant distinctions between the practicalities of push backs in Australia & the Uk.
1 Asylum Seekers in boats crossing to Australia cover long distances and therefore use actual boats - pushing a relatively robust vessel back is much less likely to cause death/injury than doing similar to a lightweight raft/dinghy.
- Australia were able to carry it out in international waters - so a) lots more space to manoeuvre, quieter & less likely to pose risks to third parties and b) no third country complaining about what's happening.
Carrying out pushbacks in the Channel offers a wildly different prospect - waters are either French or English. The French won't accept English vessels shoving rafts up their beaches.
And much more dangerous not just to asylum seekers but to international shipping.
Which was why the Navy declined to get involved.