"Mixer tap " in the UK means a tap (faucet) with one spout. Individual hot and cold taps are still pretty common.
Any hardware place should have a setup to use a hand shower with individual taps. Probably find it on the market, discount store or try B&"koo" (as they say in Norfolk) , Homebase, DIY etc. Just saw some a few weeks ago. I imagine you could bodge that into a water changer with some added hose...or at least make a filler.
If you could order just the valve (main unit) from the python or aqueon, the rest appear to be all common parts; you could engineer a system specifically for your house.
I use a powerhead and a hose to the toilet to drain. To fill I have a hose wye-d from the laundry taps...we have hot and cold laundry taps, you may only have cold.