General question:

In the UK is it customary that your prepaid card remains active when you have a communication on it (send or receive a call or a text) every 6 months or so?

Nearly all networks will deactivate the SIM card 6 months after the last change in the credit balance. Exceptions are Lycamobile (3 months) and White Mobile (2 months). In practice, most networks will also start the 6 months from the last incoming call, but this is not guaranteed by the terms and conditions. Giffgaff allows some additional ways to keep your SIM card active, making it the best SIM card for an infrequent visitor to the UK; the full list is at http://giffgaff.com/boiler-plate/terms#OurRights

I confirm it......Giff gaff is the best solution.
I find some one that send it at my home for free. I recharge and use once in England. Then i send an sms every six months and it's still alive and very cheaper

I have kept a GiffGaff card alive for more than a year without visiting UK or topping up. However, I don't think receiving an SMS is sufficient to do this currently. This used to work. The policy now states that you must receive a minimum of 4 calls of at least 10 seconds duration each. Because I am not in the UK, this would mean roaming charges if I left the SIM in an active phone. Instead, I make the calls with the SIM out of a phone, so that they are forced to Voicemail. This seems to work!

Excuse for my BAD english .... i want say that i have to SEND an sms at least every six months to keep the number with g.g. and in this year i have no problem... :)

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I've got a backup giffgaff card that I forgot about that lasted much longer than six months without being cut off. But obviously, this is not something worth risking, especially if keeping your phone number is important to you. Virgin Mobile was historically very good about keeping SIMs alive, but their published policy is also six months (I was told by a rep several years ago that the "real" policy is 365 days - but there's always the chance this might have changed).