STC is equivalent to a UK major mod approval (known as an AAN - Airworthiness Approval Note). But the UK-CAA and US-FAA systems differ quite markedly.
If you aim for direct read-across from an STC, then CAA will classify it major by default. If on the other hand you simply apply for a minor mod, to do what is in a reasonably trivial STC, then you may well get away with it.
G