My BT contract notes I'll get double data if I take a BT mobile contract... except they no longer exist.
Moreover the contract currently offers me an paid upgrade and one of the benefits is... Double Data for BT mobile customers.
Nothing in the T&Cs says this is for existing customers only.
You can get double data and a discount on EE products now.
It’s been discussed over on the EE forums here
My double data carried over to EE when I moved from BT Mobile. Also, got 10% for having BT Broadband and got a pretty great deal tbh... Reduced my bill from £35 to £22.50 and got a brand new Pixel 6a for a nominal up front payment