I just checked their policy and it states:

As part of its programme of recruitment and outreach, Girlguiding UK strives to reflect the diversity of the female population in the United Kingdom and provides resources and support to Leaders to enable them to welcome all girls and young women regardless of faith, race, culture, nationality or any other circumstance.

Spirituality

Guiding does not subscribe to any particular faith or religion and believes that every member should be encouraged to take an active part in the religion or faith of her family and community.

It seems that the they actually refuse to acknowledge even the possibility that there may be people in the UK who do not have a faith and judging by the last part, it appears that if they ever met one, they wouldn't approve.