Now I may be being thicker than a whale omelette but what other sources do you have? Why can't you just route the Sky Q though the HDMI input on the Oppo 203 and use it's HDMI audio output into the HD621? At the worst case you might need an extra HDMI cable but surely you can find one for less than the £1500 UHD722 upgrade cost???

Sources: MS600, Fidelity Audio Reference Oppo UHD-203, Roon/QNAP, AppleTV 4K
Processors: Arcam AV40, 271 x2, Lumagen 5348 with Keces P8
Speakers: DSP7200SE FL/FR, DSP5200SE VC, DSP3200 x11, DSP320.2 VOG, DSW x4
Display: JVC DLA-N7 with PK-EM2, Panamorph Phoenix Pro, 128" 2.37 Curved Silver Ticket AT Screen