Prior to buying any front or rear door you should check with your local planning authority if any approval is required i.e. if you live in a conservation area.
The installation of replacement doors only is not covered under Building Regulations, good practice would be to replace any existing doors with higher performing products.
The installation of new or replacement doorsets needs to be carried out by a competent certified tradesman i.e. a FENSA or CERTAS installer or it should be signed off by building control. You will require evidence that you have done this when you come to sell your home.