Hope you don't mind Hans, I've just added a few instructions to your batch file. So, copy the code below, paste it into a plain text editor (Notepad will do) and save it as (say) 'warp.bat'. Then follow the directions in the file.
call "C:\Program Files\FWTools2.4.6\setfw"
for %%f in (*.tif) do gdalwarp -s_srs EPSG:4326 -t_srs EPSG:27700 %%f warped\%%f
REM ***READ ALL OF THESE NOTES BEFORE DOING ANYTHING***
REM (1) Install FWtools from http://fwtools.maptools.org/. The current version for Windows will
REM install by default in C:\Program Files\FWTools2.4.6. If it is installed in a different
REM directory the second line of this file will need to be edited to reflect this fact.
REM (2) Place this batch file in the directory containing the GeoTIFFs to be processed.
REM (3) To run the batch file open a command line window in the directory containing this batch file
REM and the TIFFs to be processed. Assuming you have called this batch file 'warp.bat', at the
REM command prompt type 'warp' and press return (or you can just click on the 'warp.bat' icon).
REM (4) In line 2 of this file the output directory (where the processed TIFFs will be placed) is
REM called 'warped'. This is a sub-directory of the directory containing this batch file and the
REM GeoTIFFs for processing and it must be created before the batch file is run.
REM (5) Edit the source and target ESPG codes in Line 3, as needed. EPSG codes may be obtained from
REM http://spatialreference.org/. In Line 3 the first EPSG code is for the source projection and
REM the second is for the target projection.
Caversham, Reading, England.