I'm not sure what the problem with treating the water is here? We all drink tapwater that has been through people and animals digestive tracts, has been part of sewerage and been stagnant, has chemical runoff and been generally filthy.
Using a good filter then activated Charcoal then Chlorine will get you tap water standard water no matter the source.
From the original post, the OP appears to be planning to introduce his GF to the great British outdoors by means of what should be a pleasant bimble along some of our inland waterways not creating a survival scenario and the prospect of drinking icky water when there is unlikely to be any need to do so, seems a surefire way to put her off.
Yes, I've got a millbank bag, a filter and various chemicals that should let me drink safely from an open sewer but despite being a veteran of many UK long distance footpaths (including the Ridgeway albeit about 30 years ago) and cycle routes, I've never had to resort to taking water from a canal or other standing water other than mountain tarns etc. From what I've seen dragged up from the bottom of a canal, if push came to shove, I'd choose a muddy puddle over a canal!
I get by on about 2 litres during an overnight camp but 3 would be better. During a days hike I'm like a horse and probably drink about 4 litres in the UK more in warmer places! I sweat it out though. My hiking buddy drinks half what I do but doesn't sweat!
I recently did the Ridgeway, you know that really famous 90 mile hike that is thousands of years old, a jewel in the British crown of Ancient walks. For such a famous trail there was scant water to be had on route, two of the marked taps was turned OFF! Several publicans were Not friendly at all when we did wander off track to seek liquid refreshment??
Put me quite off walking in Britain, miserable locals with miserable weather with miserable farmers/landowners and even worse amenities!
Seriously?
Hopefully the OP's GF is not reading this thread but if she is, I can assure her that if your attitude is right, you will find the vast majority of the folk in rural areas friendly and helpful and I can't imagine many people refusing to fill up a water bottle for someone obviously out backpacking. I have a cheap folding plastic 5 litre water carrier which takes up no space, ways < 70g empty and is perfect for filling up before you pitch camp.
As for pubs, offer to leave your muddy boots at the door, remember that these guys have a living to make so buy a drink before asking for a water bottle refill and don't tuck into your own grub inside or in the beer garden - not hard rules to follow and I've never hard a problem with pub landlords despite sometimes turning up as if straight from a bog snorkelling contest.
By way of example of rural hospitality, at the back end of last year, I was walking the Beacons Way a 90 odd mile route across the Brecons and asked an elderly couple where the bus stop was in their village. They had a brief argument about what time the next bus was and because they weren't sure whether I had missed the last one (I hadn't), they insisted that I come back to their house to get the car and then drove me 8 miles to the next town to get the train!
As they say "smile and the world smiles with you!"
