The 135 rule exists. It is not invented, and it is not a misremembering of something else. It is a provision of the US Uniform Plumbing Code, and it has no application to a house in South London.
What the rule actually says
Uniform Plumbing Code 707.4 reads: "An additional cleanout shall be provided in a drainage line for each aggregate horizontal change of direction exceeding 135 degrees (2.36 rad)."
Read it slowly, because the grammar is doing the work. The subject is the cleanout. The rule requires an access point to be installed. The 135 degrees is the trigger for that requirement, not a restriction on anything.
The mistake almost every article makes
Consumer-facing pages describe the 135 rule as a maximum bend angle, saying that a drain may not turn more than 135 degrees, or that bends beyond 135 degrees are prohibited. That is not what the provision says and not what it does.
What it says is that once a horizontal drain run has cumulatively turned more than 135 degrees, an additional cleanout must be fitted. One 90 degree bend plus one 45 degree bend crosses the threshold. So does a series of three 45s. Nothing about those arrangements is forbidden. The code requires only that, having accumulated that much turning, the run remains reachable with a rod.
The word carrying the meaning is "aggregate". It is the running total across the drainage line that matters, not the sharpness of any single fitting. An article that describes 135 degrees as a limit on a bend has read the number and not the sentence.
Even in the United States it is not universal
The Uniform Plumbing Code is one of two model codes used across the US, and jurisdictions adopt one or the other. The competing International Plumbing Code takes a different approach at 708.3.3, requiring a cleanout at each change of direction exceeding 45 degrees, with additional cleanouts along the developed length of the run.
So the answer to "is 135 the rule" is that it depends which code the jurisdiction adopted. A figure that varies between two codes in one country is not a principle of plumbing.
It is not a UK rule at all
Drainage inside and around a British house is governed by the Building Regulations, principally Approved Document H, together with BS EN 12056 for gravity drainage within buildings. Neither contains a 135 degree threshold.
What Approved Document H requires instead is the same engineering outcome by a different route: bends should occur in or next to inspection chambers, should have as large a radius as practicable, and there must be rodding access at changes of direction. BS EN 12056 works similarly, specifying swept rather than square entry junctions and access at changes of direction.
Two other things are worth ruling out, because they are sometimes offered as the UK equivalent. Part G of the Building Regulations covers sanitation, hot water safety and water efficiency, not drainage falls. WRAS guidance covers water supply fittings, which is the other side of your plumbing altogether.
Why a British homeowner keeps meeting this question
This part explains why the question feels like it ought to have a British answer.
Search results for the 135 rule are dominated by US and Canadian plumbing company blogs from firms in Houston, Austin, Southlake and Ventura. A search restricted to UK domains returns almost nothing, which is itself the evidence: a rule with any UK currency would leave a UK footprint. One result serving this content sits on a Kyrgyzstan domain, which is what a content farm looks like from the outside.
The route into British search results is traceable. The claim was syndicated from a contractor blog into a consumer lifestyle site and onward through news aggregators, and that laundering is how a US contractor assertion acquires enough apparent authority to appear in a People Also Ask box shown to a reader in Lewisham. The single UK site running the article also asks what the "Maguire's rule" in plumbing is, a question with no identifiable basis at all. When a page produces one invented-looking question, treat the next one the same way.
What to do with it
If a plumber cites the 135 rule at you about a British drain, they have read an American blog and probably a wrong description of one. It is not a reason to distrust them, and it is not a reason to change anything about the work.
The question that does apply here is whether there is rodding access at every change of direction, because that is what Approved Document H asks for. A drain you cannot rod is the actual problem the 135 rule was written to prevent, and the British answer to it has never involved counting degrees.
An honest answer to a question with no British answer is still worth having.