FF may have the answer here. I have Anquet but I don't use it much since I have only the Philips navigator maps not the OS 1:50,000 and I prefer Memory Map. But I vaguely remember that when I first got Anquet, the installation of the mapping wasn't intuitive and straightforward. I think I had to import the mapping into the programme, in my case by way of a download from t'web.
You mention having bought a disk - CD version presumably? Are you certain that you've completed the 1:50,000 import process fully - if you have, I think you should see it named in the drop-down menu at the top of the screen. Note that: "OS GB Map" is the 1:250,000 free Road map that FF mentions; I think this comes with the application - I haven't bought any OS mapping and it's present in my version. But you won't get lanes and small roads when you zoom in as they aren't on this scale of OS mapping. (My version of the main application is v6.2.12, so not yet fully updated)
I think you need to achieve an extra option in the drop-down list, probably specifically OS 1:50,000 or Landranger or whatever it's called nowadays.
Hope this helps - apologies if you know all this already!