You know do what works with your financial and physical reserves so you don't have to worry about. A GP might order a test for you, but you would have to explain why. Some doctors are okay with that, most are not ( in my experience in the US).
The other thing you could consider trying is a juice fast or juice feasting, it would allow you a slower detox while helping you to eliminate pounds.
Also just eating raw. There is a girl on the web, I think she did a 92 day juice fast , but originally lost weight via raw. You might find her on Utube, plug in juice fast or juice feasting.
Lastly after eating healthy for awhile ( nutrient dense, mainly raw), you can go back to water fasting.
Short fasts are a success as well as long fasts, because you are helping your body. The key is what do you do after the fast, or the juicing? If you change your lifestyle it benefits you, if not the weight piles back.
I think I read somewhere that Paul Bragg recommended 10 day fasts 2-3 times a year. His book is popular but I think less scientific that Dr. Fuhrman.
Anyway if you are short on cash , just read the websites Chrisb1 and Fonty put up as they will provide you a wealth of knowledge and experience.
Please keep us posted I would like to hear about your journey and any detox issues that crop up.