If you wanted to do the ISTD DDE, the Anna Morgan Teacher Training College does a part time distance learning course. I believe for the practical modules there is a week you will (probably) need to travel and stay somewhere (Kent?) to attend. It is on the approved dance centre list given by the ISTD. Although if you know Cecchetti, you may want to make sure you find a dance centre than teaches ISTD Cecchetti syllabus rather than imperial ballet (mesh of French and English style). Imperial ballet seems more widespread as far as I can tell. You can find information about what the DDE entails from the ISTD website.
Regarding popularity, I'd say RAD is the most popular exam board for ballet since it started working with IDTA, closely followed by ISTD. I did ISTD imperial ballet and then added RAD vocational grades as extras (before I changed dance school to a BTDA school which is another story). I much preferred ISTD because of the emphasis on free work. You have to be really good at picking up choreography and performing it as well as knowing terminology and its definition. Lots of people say that RAD is better for precision as all the work is set (aside from the free enchaiment), but I think precision comes from a good teacher (and good proprioception), not knowing all the exercises inside out. RAD does include character, ISTD does not (but does have a separate National Dance syllabus). The good schools that I know of teach both syllabi at the vocational grade level to make up for what they each may lack.
There is also an association (and exam board) for the Russian method, but I don't know about that. I do Vaganova method now as a PhD student/adult and my teacher isn't part of it, probably because she primarily teaches adults. Might be worth looking into it.
Hope this is helpful!
