I was watching Jess Franco's version, Count Dracula from 1970 (it also goes by a lot of other titles, as is common with a lot of Euro-horror from back then), and it does a better job than most films at explaining why Dracula went to London - explains it better than the book probably, even though it is about things mentioned in the book.
What it comes down to is that he is the last of a glorious bloodline with a rich history, but basically since that's all in the past now, he feels kind of alone and like a left over. So he's up for going somewhere else and experiencing new things.
The blu-ray from Franco's film from Severin is excellent and has countless (!) extras. There is an audiocommentary with Maria Rohm (Mina), who I found out passed away very recently (last June). She's a typical filmbeauty, the kind of classic beauty you only ever seem to see with certain film actresses. She seemed to have a very lively, even frivolous personality - sleeping with crew when her husband (the producer) was away, winking at the person filming the behind-the-scenes footage. In fact, there is a bonus film, which consists of a combination of filmfootage in front and behind the camera, but showing how the film would play in black & white and as a silent film (there's only music and sound effects). It's interesting to see how things were done, to create fog for example, and there is a kind of machine which basically shoots out cobwebs to decorate scenes. Even though Jess Franco was clearly a low budget filmmaker, it's still interesting to see how surprisingly professional everything is done and it looks more money went into these kind of films than you would think.
I like the film itself and it has quite a cast (Christopher Lee, Herbert Lom, Klaus Kinski), but clearly it rather falls apart towards the end. It's either a question of money, or Franco just not being able to put on a good finale, or a combination of these probably. According to Rohm, Franco was 'on' or 'off', he could do good work but he could just as easily lose his interest in a film.
Actually concerning the book, another thing also bothers me: why did Dracula keep Jonathan Harker prison and not just kill him? There was basically no need to keep him alive - he had the papers of his new home in London - but he keeps Harker imprisoned for some reason, and even goes to great lengths to protect him from his three 'brides'. Why?