My funding for dexcom has been extended by my Surrey ccg to December 22. Even with some issues with the phone app (now uninstalled) I have reduced by hba1c to 48 where even with low carb I'd been 54-64.
The fact that I eat low carb does help the pump smooth out the small bumps that occur. Bolusing is where mistakes happen (timing and guestimating carb) - as with all pumps it only works optimally if you have your basal rate right to begin with and are disciplined re carb counting and timing of boluses. The benefit to the combination of dexcom and tandem for me is the little tweaks occurring without my awareness that are giving me a much better result.
It has been good at minimising hypos by stopping the hourly delivery when it anticipates a fall below the range.
So I'd go for the tandem pump initially and lobby hard for the dexcom funding given that the tandem iq still has a bolus wizard for you to enter your blood sugars from fsl, in the meantime. It is easy to add the Dexcom part in if you decide to invest in a trial supply via self funding.