Disrepair is different from general fair wear and tear or tenant-caused damage. It's specifically about the landlord failing to meet their Section 11 repair obligations after being made aware of the problem. If your boiler breaks, your roof leaks, or your windows don't seal properly, and you've reported it but the landlord hasn't fixed it, the property is in disrepair.
You have several routes to address disrepair. You can report it to the local council's environmental health team for an HHSRS inspection, which can lead to an improvement notice. You can take direct legal action under the fitness for human habitation legislation. You can also pursue a claim for damages through the county court for the inconvenience, loss of amenity, and any health impact caused by the disrepair. In some cases, you can use the right of set-off to arrange repairs yourself and deduct the cost from rent.
The key to any disrepair claim is evidence. Report every issue in writing, keep copies of all correspondence, photograph the problem, and note how long the landlord takes to respond.