Ive had a quick look about on the net for you. Cant really see anything out there. I did find this though
http://www.hintsandthings.co.uk/garden/baskets .htm
I would imagine that you would be allowed to hang shelves and drill into the wall INSIDE your flat. What I would do Is get 2 small brackets, fit them securly underneath your window on the inside of your flat. Then id get some really strong wire and attach that to the brackets and the basket.
If you dont want to drill you could get some free weights
http://www.argos.co.uk/static/Product/partNumb er/3008205/Trail/searchtext%3EWEIGHTS.htm
and attach the wire to them. What id also do is get some floor protectors
http://www.diytools.co.uk/diy/Main/sp-44-888-3 75-25mm-felt-gard-plus-protects-wood-floors.as p
or something similar to attach to the back of the box to protect it.
Whatever you do I would suggest that you make sure it is VERY SECURE caus obv window boxes are very heavy and dangerous.
Hope this helps : )