Though checking using the phone location seems like a reasonable answer, that would give privacy concerns -- do members really want giffgaff to log their daily location, even if it is only a daily "was in UK" / "was not in UK" flag ?
Actually that question doesn't need an answer, because it's already a prohibited method so far as roaming regulations are concerned:
Protection of personal data
Your operator is required to comply with the relevant personal data protection rules and may only use your data (which they already have for billing purposes) to check and compare your roaming usage with your domestic consumption.
So although phone location (when switched on) might be a better way to count days in UK, its not something that giffgaff can legally do and indeed giffgaff seems to be doing it precisely how the EU regs say it should be done.