I'll answer your questions one by one.

You can order wardrobe boxes from Websites like Shop4boxes, or you can get them from retail stores like paper supply companies or home retailers like Home Depot.

If you pack the wardrobe boxes with just clothes, you can expect each box to weigh about 75 pounds. There is extra space at the bottom of these boxes that’s good for light items like pillows, blankets, shoes, etc. Don’t put heavy stuff there as it can destroy the box (or destroy the poor guy who’s trying to pick it up).

The boxes can be delivered overnight (though you should try to plan your packing a little farther in advance..).

As far as over-ordering, the return policy depends on the individual company, but I don’t know if I’d let that dictate my decision. Just be sure you order enough so you don’t have to keep ordering more.

We’re not a moving company ourselves, so checking with international moving companies about these shipments. If it’s just a few boxes, try UPS or any of the larger shipping companies for an estimate.

Finally, one other thing: when you ship to London, whoever ships for you may have to repack the boxes and do an inventory -- customs requires that the contents be certified by the moving company or shipping company.  UPS can do that for you also. So before you get into the packing process, find out how this will be handled.