The values 0.35 ohms and 0.80 ohms are for run-of-the-mill metered supplies from approx 40 A to 100 A. The values were quoted in Engineering Recommendation P23/1, which was updated back in 2018 and P23/2 says something different.

This article provides a little more information regarding the replacement of the old version of P23/1, from which the rule of thumb values came, with P23/2: electrical.theiet.org/.../

Unmetered supplies ( larger metered single-phase supplies) are (and to be honest always were) "out of scope" for the rule of thumb.

Graham, thank you. I thought that Table 2 was particularly interesting. Does it matter if Ze is say, 1.0 Ω? Well yes if R1 + R2 > 0.37 Ω and your sockets are on a 32 A type B MCB. (Which is how I discovered my "excessive" Ze at home.) If I understand the article correctly, there is no hard and fast limit, just a distribution of values. But of course, I may have misunderstood!

Graham, thank you. I thought that Table 2 was particularly interesting. Does it matter if Ze is say, 1.0 Ω? Well yes if R1 + R2 > 0.37 Ω and your sockets are on a 32 A type B MCB. (Which is how I discovered my "excessive" Ze at home.) If I understand the article correctly, there is no hard and fast limit, just a distribution of values. But of course, I may have misunderstood!