I think a 3 pay-as-you-go sim only has a fiver (£5) minimum topup. Mine certainly seems happy with that! Often seems to give 150mb of bonus data when you do a £5 topup, coupled with their 1p/mb means you get quite a bit of infrequent-use non-expiring data (plus the odd call/text if needed) for a fiver
For high-use or with lots of overseas calling, you can do better than the 3 3-2-1 PAYG plan, but for lowish use (eg 100-500mb and/or limited calls/texts) it does seem to be the best UK PAYG plan at the moment.