Under the normal working hours regulations, adult employees are entitled to a single 20 minute unpaid break if their shift exceeds 6 hours. That entitlement is neither 'cumulative' nor 'pro rata'. i.e. even if you work a 24 hour shift you're still only entitled to a single 20 minute unpaid break.
However jobs which require 'continuous staffing' (as in a hospital) are exempted from the normal provisions relating to breaks. You can be expected to work without a rest break but you should then be given 'compensatory rest' at a later time.
See here:
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029451
It seems unlikely that the Working Hours Regulations can help you much. However Health & Safety legislation is entirely separate to those regulations. Under HSE laws, your employer is obliged to consider the health aspects of any working practices. You
might be able to argue that a lack of fluid intake presents a risk to your health but, now that recent research has basically rubbished the old idea that we all need to drink more water, you might find it hard to do so. (I've worked plenty of 12 hour shifts without any food or drink and I still seem to be alive!).
Chris