Consider factors like your desired water flow rate, the power rating (KW), the size of the cable you already have, shower head style, and additional features (digital controls, eco settings).
With the assumption being every electric shower we've seen (besides Aqualiser) have been in zone 1 of the bathroom, you would want a qualified electrician undertaking the work, 1) To ensure the safety of the shower where you're bringing mains voltage in close proximity to water and 2) so they can sign off their work and notify building control.
