They want your money.
Simple as that.
IF your tap water is drinkable, AS most of UK is, Forget about the filter.
I have never used the internal filter, I have connected an external filter, (because I am not happy with local supply. I had a very slight scum on my cup of tea). The filter gets rid of chlorine etc. Very easy to set up an external filter, And a lot cheaper. I would never use an internal filter,
1st they put the price of it up, 2nd they make sure that you cannot get the filter anywhere. except from them.
BUT not needed.
IF you have problems with your water supply,scum etc. Get an external filter fitted, readily available from diy stores. basically a plug in type of thing. Never get an internal filter, 10X the price.
SD, September 2011