Upvote:4
Leave your old sim card with someone at home. Ask him to boot with it once a fortnight, perhaps checking for critical messages. He can also help set up call forwarding for you after you get your travel number.
Upvote:8
If you have a monthly bill, just keep paying the bill. If you have a pay-as-you-go plan, top up your credit every month or three.
Put the UK SIM card in every once in a while and make a 2-second call. Won't you want to check your voice messages every now and then?
Above all, read your service agreement and/or call your mobile provider's service center to find out what their actual policy is.