I'm not in the UK (actually I'm in Mexico)... but... just dropping you a note of support.
I've also done long MCs and juice fasts (20, 30, 60 days), and like to have the long ones supervised by my wonderful Naturopath who has fasting experience. I have been grateful for her assistance, particularly on the very long one when I passed a small kidney calcification around day 18... a crisis that took about a week and some herbal and homeopathic support but went well.
I never thought I would fast on water, as I don't think you should have it rough! Results on Master-Cleanse are excellent, and I was even able to keep my gym routine! But for some reason, the last couple of MCs I have just slipped into a day or two of water, layered in with the lemonade. It felt good, felt right.
This time I started in with my usual pre-cleanse diet of fruits and veggies, tapering off my one weenie cup of coffee over three days and so on... drank two days of lemonade, went to water and have been there ever since (this is day 6). I keep thinking if I need a boost I will do a day of lemonade, but so far it hasn't happened. However, I am not doing the gym with the water, I do quite a bit of just hanging out and easy activity.
As far as supervision, I never have it with shorter (10-15 day) fasts, and have enough experience that I don't feel I need it. I listen to my body, keep my colon very clean every fasting day, drink plenty of liquid, whatever I'm drinking... and of course the pre-cleanse diet makes thinks so easy.
I don't necessarily take my bp, but do pay attention to how I am feeling. If I were really down, I would have a glass of lemonade, or a cup of mint tea sweetened with maple, or a cup of hot water with a tablespoon of blackstrap molasses stirred in, then see how things go.
For me the big difference between the Master-Cleanse and water is energy level. The Master-Cleanse allows my full routine, water is very very laid back. Whatever you decide, you will be fine if you listen to your body.
Anyway, a big long distance hola! Have a great cleanse.