There was a lot of discussion on that lately:
OSGB36 to WGS84 reprojection 'error'
http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon-problem-td4965339.html
https://stackoverflow.com/questions/1426941/proj-4-library-and-osgb36
If you want it very exact, use the +nadgrids and .gsb file mentioned. Here I gave some command line examples: How to reproject a raster file in QGIS with datum transformation?
If you want your data to fit to each other, always use the same +towgs84 instead of +datum. In your case, update the GDAL to 1.9.2.
For clarification: the ellipsoids airy and WGS84 do not match exactly. Therefore the +towgs84 parameters were invented. But these even change across the country. The +nadgrids takes this into account.
EDIT
Just to explain a bit deeper:
EPSG:27700 is originally only a definition for the projection and ellipsoid used. Datum shift transformations from one ellipsoid to another are listed seperately by EPSG, with their own EPSG codes. There are several transformations available from OSB1936 to WGS84, depending on the region or wanted accuracy. These are:
EPSG:1195 Accuracy: 21m
+towgs84=375,-111,431
used by GDAL 1.9.1,Qgis Lisboa/Master +towgs84:
EPSG:1196 Accuracy 10m
England
+towgs84=371,-112,434
EPSG:1197 Accuracy 21m
England, Wales, Isle of Man
+towgs84=371,-111,434
EPSG:1198 Accuracy 18m
Scotland
+towgs84=384,-111,425
EPSG:1199 Accuracy 35m
Wales
+towgs84=370,-108,434
EPSG 1314: Accuracy 2m
+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894
used in proj +datum=OSGB1936
used in GDAL up to Version 1.7.3
EPSG:5339: Accuracy 0,03m
+nadgrids=OSTN02_NTv2.gsb
the official grid datum can be obtained here: http://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/ostn02-ntv2-format.html
The three-parameter-transformations were published by NIMA in the last century calculated from measuring of a handfull of satellite stations, while the seven parameters are published by Ordnance Survey more recently.
So depending on the GDAL version a raster is georeferenced with, you might get an offset, or might not.
EDIT2
relief is on the way:
http://trac.osgeo.org/gdal/changeset/25589
So there is a good chance to havethat the seven-parameter transformation will be back in the next QgisQGIS version ;-).
