TomTom for iOS stores all the maps locally and is thus a gigantic app (1.9GB for UK and western Europe on my iPhone) and buying overseas maps is easily done, they're all in the UK app store. I haven't tried it on an iPad mini or similar Android device, but in general I've found it to be much better than the dedicated TomTom I had before.
It feels pricey for an app, but as far as I know it's cheaper than the equivalent dedicated satnav, and one less thing to carry around or charge.
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