I used 4mm needles (US 6) and DK weight yarn. (Tardis blue, white and black)
I used acrylic yarn, but if I had to start over, I'd look for a non-elastic yarn like cotton. I had to double the inside of my bag so the Tardis keeps its shape when I put things inside.
The pattern is a chart (the "+" are meant to be dots, so P on "right" side, and K on "wrong" side, no "+" means K on "right" side, and P on "wrong" side) and it is still a WIP, I'm trying to make a better version before releasing it officially (freely of course).
You need to knit twice the chart. Flap is optional but nice to have, I knitted the chart starting from row 58 and up to make the flap.
Then you knit a 14 st x 12 rows patch of white.
Then you just have to embroider the "Police Public Call Box", make side panels (or not, depending on the side you want, I made 16 stitches wide panels for the sides, bottom, and top).
Then you can make a small white i-cord to knit on top (to make the top light).
And I made a crochet cord (chain st and then sl st in each chain) to "attach" the flap to the door handles (buttons actually).
Note: The chart is for straight needles, feel free to knit in-the-round, but you'll have to adapt the chart.