You would pay £200-£300 for a domestic consumer unit ? You can get a 7/7 split board with RCD and breakers for well under £100. He only needs a breaker at the house end.
If the board in the house is RCD protected and has a spare way, then all you need there is a breaker. Then there is the arrnoured to the pool. 50m away and 5kw load either 6mm or 10mm 3 core. Dunno which without doing the calcs. Then the termination/isolator at the pool end. Breakers are around £6. An isolator at the pool end say £20. Dunno how much the armoured will be. Then there is labour. A day for me and a boy would be around £300. Ball park figures obviously.