When considering installing a new sewage treatment plant it is advisable to contact a sewage consultant. A list of accredited installers and engineers can be found on the British Water website at

https://www.britishwater.co.uk/search/custom.asp?id=6339

You can also use same website to check whether the equipment you are installing meets the current British Standards at 

Accreditation Certification certified small wastewater treatment systems for up to 50 people

Given the requirement for ongoing maintenance it is recommended that you enter into a servicing and maintenance agreement with your contractor to ensure the continued good operation of the system. Planned preventative maintenance and servicing is likely to save you money in costly repairs.