"始终如一的质量" can be translated into English as "consistent quality" or "unwavering quality".
"Consistent quality" emphasizes that the quality remains the same over time, without fluctuations.
"Unwavering quality" conveys the idea that the quality is steadfast and does not waver, adding a sense of reliability and determination.
Both expressions are appropriate, depending on the context and the tone you wish to convey. In most business or product-related contexts, "consistent quality" is a commonly used and straightforward phrase.