Buy something from a BA e-store store displaying the 'speedy' logo to get Avios credited within 5 days otherwise it's a long wait.

An online purchase of something that doesn't require posting or collecting should work from anywhere globally.

Maybe a trainticket from Trainline would work but you'll have to create an account. Beware earning rate is 1 Avios/£2 for existing account holders. Pick 2 stations adjacent on same line e.g. Davenport & Woodsmoor £2.40 (or a child fare is £1.20). There's a small booking fee. Ticket delivered by email.