Online shopping of Rakhi gifts really picks up during Rakhi season. In fact siblings separated by distance start planning for it a little early because of the distance and the time it may take to get the Rakhi gift delivered in time for the festival. Rakhi comes once a year, celebration lasts one day only and hence nobody wants to miss it. Your Rakhi is delivered one day late and your brother misses the whole fun of it. Likewise, if your sister does not receive her gift on the day, she knows the gift will arrive and the love will stay but the day is not complete.
UK has a huge Indian community and Raksha Bandhan is thus a popular celebration over there; plus, online shopping for Raksha Bandhan is a big time thing in UK.