A website of the Royal Town Planning Institute, which offers advice on town planning matters.
The regional service of Planning Aid in London.
The Planning Inspectorate is a Government agency that processes planning and enforcement appeals and holds inquiries into local development plans. It also deals with a wide variety of other planning-related casework, including listed building consent appeals, advertisement appeals and reporting on planning applications.
Run by the Department for Communities and Local Government, this site gives information about the planning process.
The Government department responsible for planning
There are also organisations that can help you to assess the non-planning implications of planning applications, such as the impact on wildlife: