You can purchase a top up outside of the UK however this can only be done by using a UK registered bank card and calling the network provider customer service team directly.
Mobile phone call credit, also known as top-up or re-fill, can be purchased over the counter in all newsagents, petrol stations and post offices. Credit can also be purchased from each local network provider and in selected Sim Local Airport retail stores.
This can be done simply by texting the relevant code assigned by each network provider. These codes are included in your SIM card pack. You can also obtain them from each network provider’s website.