The fibre based FTTC Unlimited Faster Broadband is the £21 package. It's slightly cheaper than the Unlimited Infinity 1 (£23) and also has 50GB BT Cloud rather than 5GB, so BT do try and make up a bit for the slower speeds.
Though your full estimated FTTC speed range is 5.2 to 14.8 I think it would be wise to assume the worst and that you will only get ~5Mbps. Ask yourself whether the £21 package is worth that speed increase. Any faster speed would then be a bonus.
Note that I think BT Consumer tend to quote the A - Clean estimates which is sometimes over optimistic. So they will probably tell you that you should get 8.8 to 14.2Mbps.
I don't think BT offer any cheaper 20GB or 40GB data usage versions of Faster Broadband, just the single Unlimited package?
P.S. Don't go through the normal BT Sales route. Call the UK based Customer Options Team via 0800 800 030 and they can discuss possible re-contracting packages and discounts/offers.
Why is it BT does not wish to hold on to their customers. £18 per month for unlimited broadband - £7.50 for unlimited broadband for new customers. So for every three existing customers you lose you need to have 7.2 new customers. Come on BT play the game and reward loyalty. Sky and TalkTalk are FREE......