I don't know what your friends told you, but either you misunderstood them, are talking solely about base (and see my other comment for why >100k base also exists in those roles), or were given inaccurate information. 100k GBP would be 45-60% of the credible range for senior engineer compensation at Google/Facebook in the UK (and maybe 55-70% for Amazon). That's so far outside of the standard compensation bands that other explanations are more likely.
I don't know what your friends told you, but either you misunderstood them, are talking solely about base (and see my other comment for why >100k base also exists in those roles), or were given inaccurate information. 100k GBP would be 45-60% of the credible range for senior engineer compensation at Google/Facebook in the UK (and maybe 55-70% for Amazon). That's so far outside of the standard compensation bands that other explanations are more likely.