[NFH]

With UK SIM cards, usually any balance deduction is sufficient. While outside the UK, the cheapest balance deduction is probably an outgoing text or a brief incoming call (provided that incoming roaming calls are billed per second). I have a spare Three UK SIM card with zero credit on it, and in practice I keep it active with an incoming call (obviously free within the UK) every six months, for which Three receives a small amount of termination fees; however, this last method is not their official policy.