BT Mobile offers run through various cycles and the 15GB 4G SIM is currently running with a £4 per month discount on the standard £20 for BT Broadband customers (£25 if not a BT Broadband customer) and a £80 iTunes or Amazon gift card to claim for those ordering before the offer ends on 28th October 2016. 2GB SIM is a £40 gift card, and 500MB a lowly £20 gift and no price discounts running on these two.
The change for this cycle of offers is the introduction of the Family SIM plan, i.e. a family can have multiple SIMs each with their own allowances and the price for each additional SIM is less (though additional SIMs don't qualify for the special offers. The pricing means that if paying £10/m for 2 GB SIM as the primary account, the second SIM will cost £8/m, third £7/m, fourth £6/m and fifth £5/m. The additional SIMs are all billed to a single account and the additional SIMs are on a flexible 30 day term.