The jist of this (from what I understand) is that the policy in unlawful due to the principle of non-refoulement (a person seeking asylum should not be returned to their country of origin if doing so would put them at risk of harm). The linked BBC article states this.

The Guardian article linked over here states something similar:

[T]he legal test in the case was whether there were substantial grounds for believing that asylum seekers sent to Rwanda would be at real risk of being sent back to the countries they came from, where they could face ill treatment.

“In the light of the evidence which I have summarised, the court of appeal concluded that there were such grounds. We are unanimously of the view that they were entitled to reach that conclusion. Indeed, having been taken through the evidence ourselves, we agree with their conclusion,” he said.

So, there’s still every chance that sending asylum seekers to another country could still happen in the future…