The Mira event XS is a power shower, not an electric shower.
O.P. search "power showers" on e.g. the screwfix website.
The most cost-effective solution is probably a "like for like" replacement.
Tee in lead perfectly possible, as above.
More likely, however, is that they split the supply somewhere near the first point of use i.e. the original kitchen or outhouse area.
Solid fuel fire for auxillary heating of hot water tank.
May or may not still be connected/full of water.
Suggest you get a HETAS bod ( with H004 Wet Appliance qualification) to have a look at it before you think of ever lighting a fire there!
At the wiring centre the three black cables with 5 conductors each will be standard (google S-plan+) , connected to three zone valves. You can take an educated guess on the the neutrals and earths, subject to testing. With a multi meter and Wagos and possibly some extra cable you should be able...