I wrote a guide posted here on the Landscape Laboratory blog (which requires use of the Mac command line):
The data comes as a zip file containing a folder for each major OS grid square, further broken down in many (2,800+) zip files each containing a single 10 x 10km .asc elevation file. For example:
Unfortunately, even with a SSD drive, unzipping all the zip files, as recommended in the user guide, took around an hour and just under 1gb of disk space. Luckily, there is a way to keep these further zip files compressed after the first, main zip file has decompressed and create a single, lean file of the entire British Isles for quick and convenient use in your GIS using the Terminal and GDAL.
The elevation data is ultimately stored in .asc files. I made a list of all the .asc file names stored within the zip folders by decompressing all the individual zip files then running the following command from the /data/ folder with the Terminal (OS X in my case):
find . -name "*.asc" > ~/GB.txt
this outputted a text file listing each individual .asc file. Here’s an excerpt:
To save yourself this time consuming stage, the text file I generated is available here (note to Ordnance Survey, it would be great if you provided something similar with future releases!). If you carried out the first step the unzipped folders can now be deleted. With a couple of find/replace commands within a text editor, the file paths were altered to
where ‘[path]’ is the file path to the terr50-gagg-gb folder. Note that ‘/vsizip//’ and the ‘.zip’ extension were included to allow GDAL to read the many individual .asc files whilst they remain zipped and combine them into a single ‘virtual raster’ mosaic. This edited .txt file can then be fed into GDAL build virtual raster program:
gdalbuildvrt -input_file_list ~/GB.txt ~/GB.vrt
After a very short time the resulting .vrt file is 1.3mb, and accesses the partially compressed terr50 folder that now only occupies 160mb or so disk space. Most importantly, the .vrt file loads into QGIS in a few seconds and can be used exactly like the .asc files would.
You will have installed GDAL alongside QGIS. Your copy of the GDAL build virtual raster will probably be located at
/Library/Frameworks/GDAL.framework/Versions/1.11/Programs/gdalbuildvrt depending on the version number.