It would surely be remarkable if people hadn’t been intrigued by the hot springs at Bath since the midsts of time. Cunliffe and Davenport, in their 1985 ‘The Temple of Sulis Minerva..’ confirm that mesolithic flints were found beneath the temple when it was excavated.
They also mention that seventeen pre-Roman Celtic coins were found near the spring. They were ‘fresh and unworn’ in appearance so it’s doubtful they were old coins thrown in later. In that case, these are the earliest votive offerings found.