David, I fail to see where a water filter will improve what is already, in the UK at least, the best water supply in the world.
Questions to ask yourself are,,Do you use a special filter at home?
If not why would you suddenly feel the need for one in your caravan?
Unless you are meticulously vigilant and change the filter very frequently, and not just once per year, you are introducing a superb breeding ground for all kinds of bugs which you really do not want or need.
Then ask if the cost is worthwhile? I would suggest it is not.
Now ask why manufacturers hav ealmost all stopped fitting filters, apart from being a cost cutting exercise, it is because you really do not need one.