DB25 to 54 pin is not available in the UK. You could ask nicely at Cooper BMW parts if they can add it to their system but don't hold out much hope.

It's also close to £60.

You could make one if you are any good at soldering.

I have no idea where your particular car goes, but I'm thinking you are looking at a dash-strip down to find and wire in the audio, power, ibus, mute blah blah... I can't believe you have a loom laid but it is not plugged in! Is the fuse in place? The colour codes for the wires is on the page with the shcmatic for the U407. You have a typo in the telematics part number.