is it okay to put socket in sink unit

Take it you mean fit a socket in the cupboard below the sink? - yes this is permissible but the socket should be accessible and mounted so its unlikely to get water drips on it should the waste or other pipe below the sink leak.

Yes you can house a socket outlet in a cupboard under the sink. However you cannot have a socket outlet feeding an instantaneous water heater or Instant boil tap if that was your intention? regards,

Yes if it's on mcbo and ip65 rated

I would house it in another cupboard if possible, failing that I would fir a waterproof socket outlet and ensure RCD protection.

Avoid if possible but fine if it's accessible. Just make sure it's protected by a RCD/RCBO .