Is it just constipation which is leading you to believe that there is an obstruction? Constipation can be caused by other things including a high requirement for Choline which is required for liver detox, which tends to increase as digestive permeability increases, leading to insufficient to produce acetylcholine which is required both for memory and for stimulating the bowel, hence constipation. There is a remote chance that there could be an obstruction but I suspect that what you are getting could be caused by other things which are not as serious. Watch out for any bleeding. Taking things like senna is not a viable solution IMO. If things do not clear up then it might be worth seeing your doctor if you can get an appointment, waiting for weeks if yours is anything like mine.
I am not a fan of British tap water because of the fluoride con ie that it is beneficial to teeth when even the BDA have stated that there is no benefit and that having it in toothpaste is sufficient. They dispose of that crap from chemical companies into water supplies including Severn Trent water. Fluoride also affects fertility which is the real reason why it is being added to so many countries water supplies to slow population growth which certain groups have big concerns about and want to reduce. Where fluoride has been added the child births per women drops below the 2.2 needed to maintain the population.
Chlorine and chloramine are more things that I dislike. They do help a little to keep water clean even if not 100% effectively and there are better methods but which are more expensive.
Reverse osmosis is about the best and adding distillation afterwards improves it further. Distillation does not remove everything and nor does reverse osmosis but both together removes just about everything except for H2O.
Bottled water depends upon the quality, what is left in it. Is there any chlorine or fluoride, chlorine in particular you do not need in your colon IMO. Avoiding fluoride is beneficial. Fluoride damaged my teeth and bones BTW. My teeth have dental fluorosis and my dentist lied to me about what is was, insisting it might of been caused by a high temperature/fever. Utter rubbish. I developed brittle bones too but that has lessened more recently following some intervention. However I still have far too much fluoride in my body. It affects the pineal gland as well.