You appear to be looking at either 'pay monthly' SIM cards or at certain special offers associated with some PAYG products. A normal PAYG SIM doesn't require any regular top ups.
All SIM cards will expire if they're not used over an extended period of time. The phone companies have to impose such a restriction so that they can reallocate unused numbers. (If they didn't do so, they'd soon run out of numbers).
Vodafone, for example, will disconnect the service to a SIM card if it hasn't been used for a chargeable outgoing call for 180 days. However the clock starts running again if any top up is made within that period. Further, once you've been cut off, you've still got 90 days to let them know that you want your service reconnected (free of charge) and your credit reinstated (with no deductions).
So as long as you make just one call (or top up your credit) every few months, you should have no problems using a Vodafone PAYG SIM.
However, if you want a PAYG SIM on the Vodafone network, don't buy it with the Vodafone logo on it. If you do, you'll be charged 20p per minute for calls and 10p per text. You can buy a Vodafone SIM which will only cost you 8p per minute for calls and 4p per text. It will cost you a fiver but it comes with £10 of credit on it! As I've indicated, it's actually a Vodafone SIM but it doesn't have the Vodafone logo on it. So where you can you buy this bargain? See here:
http://tinyurl.com/346swv2
Chris