Hm. That seems somewhat unscrupulous of them, since the filter tends to wear out first, and since absence of filtration seems to lead to solenoid valve sticking open.
If you're up for some stirring, you could write to them and point out that they're not providing spares for parts that are wearing out within the lifetime of the shower. Mark the letter "cc Trading Standards", and see what happens.
I don't know of anywhere that sells them separately, but that doesn't mean you won't find somewhere. You could try "the Shower Doctor" on 0131 538 4343, for example.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local