your RT-AC3200 is a dual core 1 GHz processor, the RT-AX88U is a quad core 1.8 GHz processor with wifi 6 standard, so a little more future proof.
Also, try this link to so what synch speeds are available in your area –
https://www.broadbandchecker.btwholesale.com/#/ADSL
https://www.sky.com/shop/choose/add-broadband/product-selection?irct=BBSpeeds-Hero-swcb
My thoughts:
As you connection has a low synch rate/speed, I would opt for a Broadcom modem as they synch higher than other modem chipsets, something like a BT HG612 from Ebay for around £20 if you are using ADSL etc…..
If there is a faster isp service/package available, the the DrayTeK 130 modem and is a good choice at higher speeds and it shows line stats etc,, (alas not Broadcom DSL chipset)
Then use the ASUS RT-AX88U set up to run on Sky credentials (easily done) and then have the RT-AC3200 as a access point or use AiMesh etc. (should work OK)
Some re-configuration required, but a good use of the kit you have and removes the Sky router which is far from ideal 
is the QoS for the RT-AC3200 and AX88U that different, if they are I assume this is just because the firmware for the AX88U is less mature ?
There is also another firmware suitable for ASUS routers called ASUSWRT-Merlin, and is worth a look – https://www.asuswrt-merlin.net/ as this will enable the use of SKY MER DHCP option 61 (MER) if you are using PPPoE etc.
A lot to consider 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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