Yup, Britpart seals ARE that bad.
Britpart supplies several branded seals, and will either denote the part with a "G" suffix, or list that it is of "OEM" origin. The Dowty seals are good quality, and will show a makers mark, as will the Cortelco, or other branded seal.
Britpart standard seals are often of Chinese origin, without a brand name package or stamp. Britpart Chinese could make an anvil that would break. If the job is worth doing at all, and it isn't a vehicle you're selling, use branded, OEM, or Genuine parts. That applies for all engine internals.