Hi Anne. I think your plaque is most likely by Colin Heaney as suggested. The common dolphin design that turns up everywhere (also shown below) is not the only type he made. Another fairly common type shows two whales, mother and calf. I don't recall seeing your design before, however.
The one in the photos is about 14cm max diameter by 2cm thick, so similar to your specimen. The photo of the back might be the more interesting image. It shows other similarities to yours.
That one is signed and dated 1993, and it has two stickers. Contrary to the advice you received, almost every Colin Heaney piece that was intended for separate retail sale is signed, and most of them from the late 1980s to the early 2000s originally had a sticker as well. The exceptions are mainly things like individual flowers sold as a bunch in a vase or small pressed items such as pendant charms.
His studio was near to where I live and such things turn up in local markets. I'll keep a lookout for your design. I once answered an inquiry here 7 years later. We don't give up!
Trevor