Don't know if there is much to choose from these days, but stick with a 'proper' name brand and buy from a good plumbers merchant rather than your local diy warehouse.
Mira are one of the good ones.
before you purchase you need to ascertain your cold mains working pressure, as thes powerful showers need at least 1.25 bar to let them work usually.
Also consider the cost of larger cabling and mcb fuse etc. It all has to be larger for this size shower.
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