I live in UK too and I am fully aware of the Consumer Protection Act and it only applies if there were a defect in manufacturing or a known inherent defect in the batteries. There is no evidence of that in the public domain.
You have 6 years to make a claim in court, but be warned there is a clause which allows the retailer (you must have bought it from Apple to claim from them) to subtract any years of use from the expected life of a consumable or the product itself.
Most people would expect an iphone to last 3-4 years and a battery for 18 - 24 months.
If a phone failed completely in 18 months you might win a claim in a UK court for maybe 1/3 of the cost of replacement if you won the case.
So if say your phone is 3 years old you have no reasonable expectation of even partial recompense for a new battery, because 3 years is greater than 24 months.
Under international Apple warranty the battery is not warranted but if you have a better barrister than Apple then by all means put in a Small Claim in your local county court. You would have to prove that an upgrade caused the defect with technical evidence and the judge would decide between your evidence and that of Apple, always assuming the judge accepted the case which is unlikely with the age of the device.
Now please carry out the two tests I indicated then come back, preferably in a new thread..