Have you had a chance to have a look at the DIY Projects area: https://www.diydoctor.org.uk/projects.htm There are several projects relating to Central Heating and Radiators that may be of some help to you.
Any concern about the flow and return pipes can be removed by fitting a bi-directional TRV on the new radiator. Modern TRVs are now bi-directional and can be puchased from most heating suppliers.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!