Yes, it can be done, but only if the plinth heater is specifically rated for bathroom installation and complies with the bathroom electrical zones under Part P.
In my experience, the biggest mistake is people fitting standard kitchen plinth heaters in bathrooms where humidity and condensation...
The taped choc block isn't the best practice. Personally, I'd ask the electrician if a small Wago/junction box solution could be fitted instead, for a cleaner, more future-proof install.