Not a fan of Airbnb.
Try Booking.com. You'll be 5 adults. You can look for apartments specifically and refine by all kinds of criteria. Ignore the stars which aren't applicable to apartments.
Pay attention to the ratings provided by people who have actually stayed in the places they review. I use 8+.
Make sure there are separate bedrooms, not just sofas that make into beds. Don't be surprised if the listings show 2 apartments priced together for 5 adults. It's not going to be cheap.
Look closely at the pictures to be sure it's a place of interest. Some things aren't searchable, like what the shower is like.
London has lots of "aparthotels." Some of those are more flexible with providing twin beds, but they might not have a space appropriate for all of you to cook together, to sit down to eat together or to sit in a living room area together. Again pay close attention to the pictures.
If you haven't used Booking.com before, you will have a bit of a learning curve. Enjoy the research.