I went with BT because they have a cheap 1GB plan with unlimited calls and text, and I basically use the phone for travel info and an occasional eBay bid so don't need huge capacity. It's going to cost me another couple of pounds a month for a slightly better service than I was supposed to get from Freedompop, but that won't break the bank. Should get the new SIM tomorrow or Monday, and be be back in business next week.
In the meantime, apologies to anyone trying to reach me - the VOIP part of it was working so I don't think I missed any calls or texts at home, but there might have been something when I was out. And apologies to anyone who got a Freedompop account on my recommendation!
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