My advice would be to contact the App Store / iTunes, tell them you are not satisfied with the app because it doesn't work, and ask for a refund of your £100.
Then buy the "UK Map" app, which is
SUPERB. You pay £6.99 for the app which includes a 1:250,000 road map, and you can then download as many 1:25,000 and 1:10,000 "tiles" (each 5 km x 5 km) as you want. The killer feature is that all the tiles are
FREE!!
The topograhical detail is superb, and in 1:10,000 the street names are fully searchable by postcode, name etc. Other features: altitude in feet/meters; compass; lock locator onto centre of map; 10-figure grid ref and precise lat/long; choose hill shading direction and density; all scales merge seamlessly from large -> small and back again when zooming in/out; and of course, it works offline (i.e. without a 3G signal).
I can't speak too highly of this app. It is terrific value, and you don't have to download more tiles than you need, so you aren't using up more memory than you have to. You can delete tiles when you've returned from your trip, then reload them again whenever you like, still free, as often as you want. You save a fortune compared to buying OS maps or Outdoors GB. Go for it!