Considering Five-Star doesn't import products to Europe so the store has to go through other channels, I don't think having the price double what it is in the US is that terrible.
The alternative of having a bottle shipped from the US to UK would be significantly more expensive. If one uses RO or distilled water to mix it and reuse the solution, that 23 pounds can become a pretty good deal. I've still got the same 32oz. bottle of star-san I started brewing with 2.5 years ago and I've got plenty left. Even if I paid 33 dollars for that bottle my sanitizing costs would still be nominal. If the OP wants to use star-san I dn't see a better alternative.