Just a heads up. BT mobile £20pm for 20gb for BT customers, 12m SIM only including extra speed 4G (EE network / BT is EE!).
We took a similar deal some months ago and can confirm that Amazon voucher and TCB do pay out. Amazon voucher £90 and cash back £52.50.
So you pay £20pm but receive £142.50 in cash back and Amazon vouchers, making it £8.13pm effective for unl/unl/20gb.
Other deals are available but have been notably better in the past e.g. We actually have the 4gb for less than £1pm effective - so if you are after that keep an eye on the vouchers and cash back aligning on that deal.
Hope this helps someone :) can confirm voucher and cash back pay out from first hand experience.