Seems an eminently reasonable request. In practice, a wifi access point power such as a VM/Plume pod, a standalone wireless access point, or a mesh system unit, energy use varies depending on how much activity it is doing, and how efficiently it operates. The 15W figure from Plume is a peak figure, Plume's web site suggests 4.5W in idle mode and 10W average. That 10W is undoubtedly an arbitrary figure rather than accurate for your particular use, and seems a bit high to me but it'll do for logic purposes. I have a suspicion (that I can't prove) that the Plume pods are slightly more power hungry than a normal mesh system, but we're talking in the 1-3 watts range. Partly that's based on user comments that the pods run hot, partly on the understanding that the Plume system involves rather more processing and data exchange to run their real time intelligent wifi.
Your hub on its own uses around 12W average, around £36 a year. A hub and one pod at 10W will be using about 22W, which at current electricity prices is around £65 a year.
If you had three pods, then it's 42W and £125 a year for 24/7 running. The real energy hog is the hub, because the DOCSIS signal processing used for the cable data connection is very energy intensive and has no low power state, so (generalising across VM hubs) it is always running at around 11-13W. The ONT of an Openreach FTTP line is around 3.5W for comparison, although you'll need to add on the power use of a router or mesh system to compare. Until recently I was running a VM hub (12W) and two Deco M4 mesh units at say an average of 7W each (based on reported tests of these systems, hence my suspicions above), so costing £77 a year. My new Openreach setup of the same mesh and an Adtran ONT is about £54 a year.
For people with smart home gadgetry, that may involve regular device polling and small but frequent data transfers, that may prevent any wifi setup going into a standby mode, and materially increase average energy use on the wifi side of things. In the longer term VM will be moving to a passive optical network that will bring customer energy use more in line with Openreach connections, but we're talking a good few years before that becomes a reality in customer's homes - it requires the rebuild of all of VM's network to FTTP and the retirement of DOCSIS. In newer built areas representing about 20% of their network, VM already have FTTP, but because they still use DOCSIS hubs there's no energy savings.