I bought a DVD box of series 1-3 in last February. The DVDs have neither Japanese subtitles nor Japanese dubbing, therefore it took long time to watch the stories and it was useful to improve my reading and hearing English.
There are helpful homepages about 'Downton Abbey' such as;
Downton Abbey Wiki (ダウントンアビーのあらすじ、ビデオクリップなどの紹介)
Downton Abbey Episode Scripts (ダウントンアビーの全セリフ)
When I visited UK last October, the season five had started and I watched one episode on a TV at a hotel in Torquay. It was really a fun.