Which UK SIM cheapest to activate from abroad just to have a UK number?

[NFH]

Originally Posted by international

As I understand the minimum top-up is GBP 5 (for all carriers)

It's only 10p with Orange (in 10p increments), but the only want to buy these micro top-ups is in Orange shops (mostly rebranded now to EE). Another advantage of Orange is that you only need to receive a communication every 6 months in order to keep the SIM card active.