You refer to a Light UAS, is that what you really mean? As per CAP 722 a Light UAS is one with mass (excluding fuel) 20+ -150KG.
If you intend using something 20KG and less, it is a Small Unmanned Aircraft (as per CAP 722) and the advice given on previous posts is good. If it is 20+KG you will get into the issues of design/build quality and will need to employ the services of a National Qualified Entity (NQE) recognised by the CAA to review the design/build quality and make appropriate recommendations as to whether the aircraft is fit for purpose prior to the CAA issuing any Permission or Exemption.