You can become a Basic member for free, as a Basic member you can create a profile and search for local members.
If you want to message borrowers, you will need to become a Premium member. Premium membership is a yearly fee of £44.99 for Owners £12.99 for Borrowers and includes unlimited messaging, insurance (full terms), safety checks and a 24/7 Vet Line.
After that, there are no other fees involved. Matches are dependent on location and availability so you can use your messages and 'Welcome Woof' to work out an arrangement that suits you both.