Let me try to answer your questions in short!
1). Firstly, whether you have a masters or not, a PhD is an independent programme. Most academics in Britain are those joined PhD programmes straight after their first degrees. So a masters is neither a requirement, nor it matters. Besides, you will be qualified with a PhD before your friends with Masters start to look for PhD positions. Consider you are lucky!
Secondly, if the subject is familiar to you, turning down a scholarship is not something I would advice. They don't come easily nowadays!
2). Nobody can predict whether you will like it in 3 years time, but if you get into a programme, you will see how much interesting stuff is there in this world that you don’t know and that you can learn. You can choose between interesting and uninteresting stuff and make the programme interesting. You have the choice in an independent programme . Also,your prof will only like very interesting stuff.
3.) If you know the practical side, you can try to develop theory or find it in books and compare them. But you will only know what you found practically and how to interpret it. But if you learn and develop theory, you will know everything about it first and then whenever you find practical applications, you will know how to interpret/describe/explain and solve them. Acquiring theoretical knowledge is a way to rule the world!
4.) By choosing one topic, you will not isolate/forget anything else. This PhD will only be a passport for you to explore the entire universe. Once you have a PhD, you are free to do whatever you want. Before you do a PhD, going to study other branches is a struggle. My advice is, get your PhD from where you can and then the world is your Oyster as they say!
5.) If he is frequently travelling or busy, you will have more work to do, tight deadlines to meet. You will be driven to work ontime because you will be prepared to discuss your problems whenever he is around.
All in all, you have nothing to worry, but just to take it up and get on with it. Good luck!