The 71X in Chelmsford is also mainly double deckers (off the 70) but I agree this low bridge makes the town a bit of a desert.
Agree the 70 to Shrewbury sometimes has deckers.
I got a decker on the 18 from Much Wenlock to Telford in October 2017 but I don't think Arriva has an allocation there now.
Yeovil appears to occasionally see deckers on the 54.
Hereford almost nothing but Sargents of Kington has one or two and I think they come into Hereford on a scheduled run, at least in school terms.
Much of South Wales is without deckers, eg Merthyr, Caerphilly, Blackwood, Aberdare, Pontypridd although there is the odd one now in Cwmbran.
Bridgend agreed, also Llanelli, Ammanford, Carmarthen and all points west. Just a handful in Mid Wales and North West Wales, none in Caernarfon (odd T2s??).
Stockton? Occasional deckers on the X12 but not much else, none allocated locally. Middlesbrough only on the X9/X10 and X93 I think.
They are pretty rare in Sunderland and South Shields but there are some routes (56, 26/27).
The main operators in Portsmouth and Southampton do not have any deckers but they do come in on the 700 to Portsmouth and several country routes to Southampton.
Stevenage? Can't think of anything here which might be.
Stafford? Odd 101s perhaps.