Most services and truckstops have showers which can be used free of charge. It is difficult to be any more specific because the details vary by operator, so you are better off contacting them.
In July 2012, Moto controversially started offering shower facilities for customers parking overnight only. They claimed that this followed repeated and extreme vandalism.
The cleanliness of shower facilities has long been a bone of contention for professional drivers, although the operators claim they are refurbishing them as quickly as they can.
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