There's a lot of nonsense talked about contaminants, particularly on environmental degree courses. Most of London's tap water was supplied in lead pipes 50 years ago and schoolchildren were allowed to play around with serious quantities of mercury and they're not all dead yet. I suppose if you plan to live for many years in a patch of wilderness you might like to know that it's safe to drink the water, but transient exposure is a very low risk. You'll have just as much pollutant in tap water. More probably, with all the additives they put into it and the hormones that sneak through.
As for giardia, it's another of those issues designed and highlighted to frighten. Like many things it affects different people to different degrees, but most people get over it quickly and with mild symptoms and generally become resistant to it. In any case research published in Wilderness and Environmental Medicine in 1995 cites giardiasis as generally not being spread by contaminated drinking water. Of over 34000 cases investigated less than 20 were water related of which only 2 were thought to be while camping. The most important thing is to keep hands and food containers meticulously clean when in contact with food. It is not as widespread as the scaremongers would have it and in any case you're more likely to be infected by having a pet around the house.
The biggest hazard is viral, and filters will not protect effectively against it. Probably not a major issue in the UK, but if you want to be certain then just boil your water. My rule of thumb is to filter down to whatever level a Millbank Sock can achieve, boil, cool, aerate then drink. Aerating your water makes it taste so much better than freshly boiled water. Obviously be careful where you collect the water, animals drop all sorts of stuff. Rainwater can be drunk untreated until it hits the ground.
