Ask for a quote first cos it could cos you a fortune.
If you want a 13a supply you will need armored cable that needs to be burried 3ft down.
This can then be connected to the consumer unit (make sure he fits a RCD breaker).
But he may want to fit a slave consumer unit in the Greenhouse then run a spur out for the light and one for the power.

I would think about running a Solar light, Sun powers battery and Batt works light. and decide how often I needed the power out there. I may just run a splashproof extension (plus RCDs) from the Garage or nearest point.

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