Yes. Not to rub it in, but this is why booking two one ways is usually better than booking a return.

Added to that there are a few routes back to the UK where the actual taxes, fees, and surcharges are actually less than the £17.50 cap. If you book a return starting from the UK you will be charged 2x£17.50, whereas if you book two one ways your UK-Destination will be £17.50 but your Destination-UK may actually be less than £17.50.