All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible to perform ablution in the toilet since there is no evidence which prohibits it. However, it is more appropriate not to perform ablution in the toilet as it is disliked to mention Allaah in it.
It is desirable for the person who performs ablution to say "Bismillaah" (in the Name of Allaah) at the beginning of ablution according to the majority of the scholars . However, some scholars are of the view that it is an obligation to say "Bismillaah" at the beginning of ablution.
It might be that this is the reason why it is disliked to perform ablution in the toilets as stated by some jurists .
In order to be able to say "Bismillaah" and to avoid the view that it is disliked to mention Allaah in the toilet, you should build a separate wash basin for ablution outside the toilet.
Allaah Knows best.