The motor industry, like many others these days, works on a Just in Time delivery basis. They have for many years now ordered parts as and when required, perhaps holding a small stock locally (i.e. in the UK). But this has all broken down through Covid, shipping delays and obtaining raw materials, manufacturing and supply due to the war in Ukraine. It's not just DPFs; many parts for all types of vehicle are in short supply and manufacturers have cancelled production of some models or trim levels, and deleted components from new cars so that they can continue manufacturing them to some extent. Airbag control modules for some vehicles, which can fail due to low battery voltages, have been on a 3-9 month lead time for quite a while now, as many failed whilst cars were not being regularly driven through Covid and existing stocks were exhausted.
If you feel capable of removing and refitting the DPF yourself, the following procedure is one that was regularly practised on a fleet of recovery trucks operated by a garage I used to visit regularly during my work. The DPFs on many of their trucks used to block regularly due to frequent London driving conditions. They removed the DPF and left it soaking in a container filled with cheap Coca Cola for 24 - 48 hours, then reverse flushed it with a jet wash. It's not a recommended repair routine, but it seemed to work, and it might be enough to get you through the MOT.
The car should also be capable of carrying out a forced DPF regeneration, which attempts to burn off excessive blockage in the filter. It has to be initiated using diagnostic equipment though, and is a rather harsh approach as it involves the car sitting with the engine racing at high rpm for 20 minutes or so, which can appear quite worrying and will certainly not endear you to any neighbours. An oil change is usually suggested after that routine as there is a good chance that the engine oil will have become polluted during the process.