BT SIM only deals ... Very cheap deals with extras.

[Kingy]

We are running on it now. I can get the £90 Amazon voucher in a week. (It's delayed to ensure you don't just activate sim(s) then cancel for the deal). 

It certainly seems pretty cheap compared to all the other deals I've seen.
Good app support too. I've put a BT 'Mobile' app on each kids phone and it shows them how much data they have left of their 6GB, when it runs out and any 'extra spend' since the last bill. No excuse for anyone to go over their data limit. (Especially with 6 gig!). And as I've got another app. covering the full account I can see any phone on the plan. 
You can put age restrictions on too, so although it's a 'family' account the kids don't have the authority to order anything by accident.

First impressions are that the support (apps) is far superior to Tesco Mobile. The Tesco app is crap (I tried it) and their preferred method of contact (online chat) is shockingly bad. I've had to drop chats as I've had an opinionated idiots telling me what I need whilst ignoring what I want. Whilst I'm no fan of BT they do set things up well.