a bag with clothes from Brussels to London

The cheapest option apart from getting a friend to do it is to use a shipping broker. Have a look here for some ideas and get a quote:

http://www.moneysavingexpert.com/shopping/cheap-parcel-delivery#internat...

I've used Worldwide Parcel Services but only for about 10kg. It was far cheaper than the post or going to soemone like FedEx.