Well, it depends on how important the short London trip is to you. If you are going because you want to see specific shows that may not be around later, I would choose London. However, IIRC it seems like you have already pushed back the WDW trip at least once during the planning stages, so if London can wait, I would go ahead with your Disney trip as scheduled. Since you live in the UK, London is easy to get to once you return.
I can understand that you may have a lot of people on these boards telling you to go to London, because for it’s a big deal and a rare occurrence. We can’t just hop on a train and be in the city in a few hours like you can. So you need to think about which is more important to you, and then move forward.