Building Regs Approved Document A Bottom of page 35..
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdfSuggests that strip founds should be min 0.45m to the bottom to avoid frost unless on clay when it's 0.75m
The minimum thickness of concrete is detailed in Diagram 23. It depends on the width. eg The thickness T should be greater than P in the diagram.
Probably ok but the BCO will normally want to see the trenches before you fill them and he may have other ideas.