Sanantan and Akal are synonnomous.Both means eternal.In Guru Granth Sahib Ji,One verse of Of Bhagat Kabeer Ji mention the word Sanatan.(Das will later give you the detail of page no).
In Sarbloh Granth,Sanatan is another Name given to Akal.
As per as Sikhs who call themselves Sanatan,They use it in the meaning of Traditional,Sepcialy in UK.
Das repeat one unique thing of them.As per them Dhrama or rightousness is eternal or Sanatan.Khalsa order was there in force since world was started.And Akal started it. Gurus revivied it again(as per Vachitar Natak all prieivious Avtars failed to do so).
As per them Ram,Krishana,Jesus,Prophet Muhamud(PBUH) all were Sikhs of Gurubar Akal(God as a guiding force).So all faith converge at Khalsa order.
Thanks Vijaydeep Bro, I absolutely agree, Sanatan Sikhi existed from the beginning of time and was simply revived by our Gurus, its why I see so many similarities between the appearance of earlier people who were very spiritual and Sikhs even now.