My understanding is that current historical linguistics views Basque as a language isolate. It cannot be connected with any other languages using the comparative method.
This does not mean that it may not be related to other languages, it just means that there is no
evidence for it at this time.
There are hypotheses that Basque is related to Georgian or Caucasian languages, or that the
ancestors of the British spoke a language related to Basque. However, these are not scientific because they do not use the
comparative method. In particular, genetics is not useful for linguistic classification, since there is no correlation between one's language and one's genes.