The way I would do it would be without a batch-file, just directly in the "dos box" console (cmd.exe):dir /B *.tif | gdalwarp -s_srs EPSG:4326 -t_srs EPSG:27700 %%f c:\out\%%f
This falls over with an error message:
ERROR 4: `%%f' does not exist in the file system, and is not recognised as a supported dataset name.
I don't think Hugo's suggestion is going to work. The first part simply creates a list of all tiff files in a directory, but there's not telling the second part what to do with them.