Thanks for the info. I had posted elsewhere about there being a 100 GB offering, but because the sales queue was lengthy, I gave up before getting a firm price quoted (I had asked if Three planned to offer 250 or 500 GB to compare with EE, and told no, on web chat, but then the sales person said to phone or visit a store for 100 GB deal "for about £30".
You've now confirmed the actual price, and thanks for the additional info on why it might not be available nationwide. I hadn't thought of that explanation for it not being shown on the website (but I suppose it avoids ASA complaints that it isn't available everywhere)... .
If you run a business, have a second ISP and backup web hosting...