Hi Charlie,
Mine is a 350h with preimum pack too, I drive between Richmond Park and zone1/2 a few times a week so experienced the same slighly disappointing mpg when in very heavy traffic in/around central london.
My experience is that there is a very specific condition where the 350h excels in terms of mpg, not too fast and not too slow, with a reasonable amount stop and go so the regen brakes can recharge the battery, and ideal ambient temperture of late teens - 20s.
I regularly get around 60mpg from zone 1/2 to Richmond, when the condiotion is similar to above; a good 15 miles trip in the usual London traffic. (I had 76mpg once over 15 ish miles on a peaceful afternoon from Euston to Richmond!)
It doesn't like very heavy or constantly slow traffic in the 20 zone, so London zone 1 in other words. Where regen brakes couldn't do much and worst of all, when the traction battery runs low enough for the engine to fire up and charge it while you're stationary in traffic, this really hurts the mpg in my experience over the last 8 months.
Cold weather obviously hurts mpg, especially when on short trips where almost the entire trip is spent on warming up the engine/cabin as quickly as possible, you then arrive at your destination before or just after this phase is over, ending up with a rubbish mpg.
Your mpg in central London over the last 2 months is probably in the wrost condition you can ever throw your NX in, low temp, short/slow trips and new engine... I'm sure as we approach summer you will see more impressive mpg to come.
On a side note I'm surprised to hear that you only collected in March when you placed your order in Nov-21. I believe we have identical specs bar the colour(350h with premium pack with link pro screen), I ordered after xmas Dec-21, and the car was ready for collection mid Aug-22. Did you order from Lexus Edgware Road out of interest?
All things considered, I feel the NX is a fine car, and even with the mpg at its worst, is still a respectable mpg for the size and comfort that the NX is, I trust most members here would agree.
Regards
Lok