It also depends on what you need it for. Keep in mind that in the UK most things just require a solicitor to certify them, which is often sufficient. For instance, this is what we did when my husband couldn't come to DDs passport app interview. They needed a notarized form saying it was okay for her to have a passport at the embassy, and it was sufficient to have our solicitor give it his certifying stamp.
If it's a US form that specifically needs notarization, the links you've been given should be helpful, but yes, it is very expensive to find people who will notarize in the UK.