Hi Maciej,
Orf published a custom workflow to do exactly what you want. It can be found here:
REST API
We currently use it to communicate with some custom web services. The API Response is captured as a string output variable
I don't know if it's the way our services are developed, but I've noticed that UCS-D will populate that Output variable with the entire HTML response if you use a GET instead of POST in the web service itself. The inputMethod doesn't seem to matter.