4mm² is pretty heavy, that is the equivalent of 11ga wire (4.17mm²).
12ga wire is actually 3.31mm².
Also, that appears to be £17 Each, not per pair, but I can't be sure of that. Even £34/pr is very reasonable.
For the USA market, here is the equivalent -
https://www.amazon.com/GearIT-Premium-Braided-Speaker-Plated/dp/B071NNZV2N/