Why is Ekadashi on a different date in the United Kingdom and India?
Because Ekadashi depends on when the sun rises where you are — and both days are correct.
Ekadashi is the eleventh tithi, a lunar day defined by the angle between the Moon and the Sun. That angle changes continuously, and the tithi begins and ends at a single moment worldwide. What differs is your sunrise.
The observance follows the day on which the Ekadashi tithi is in progress at your local sunrise. When your sunrise falls on the other side of a tithi boundary from your family's, the two of you observe on different days — both correctly.
Worked examples
London
London and India fall on different days for 13 of 49 Ekadashis in 2026–2027.
Leicester
Leicester and India fall on different days for 13 of 49 Ekadashis in 2026–2027.
Birmingham
Birmingham and India fall on different days for 13 of 49 Ekadashis in 2026–2027.
Manchester
Manchester and India fall on different days for 13 of 49 Ekadashis in 2026–2027.
So who is right?
Both. This is not a disagreement between calendars; it is the same calculation applied in two places. Traditionally, you observe according to where you are.
If your family observes together across countries and you would rather match them, that is a family decision, not a calculation one — and it is a perfectly common choice.