Freedom passes validity has to meet the minimum requirements of the English National Concessionary Pass scheme. This means it has to be valid on all buses in England between 0930 and 2300 on working days, and all day at weekends. That is the only legal requirement.
In particular, passes issued to residents of Slough, Maidenhead and Reading will not be valid on the TfL-Cross-Liz services passing through their area.