Saleem is still putting the cart before the horse. He doesn't know the rating of his cooker, he thinks the shower might be around 10kW, he's intent on purchasing the CU himself, without consultation with his phantom electrician. He's more concerned with getting an apparent bargain, when the few quid he may save is negligible in comparison with the cost of the job he seems to be contemplating - and buying the wrong thing may ened up being a costly mistake anyway.
If you genuinely do have an electrician ready to do the work (and I really don't believe you do) then LEAVE IT UP TO HIM.
If you genuinely do have an electrician ready to do the work (and I really don't believe you do) then LEAVE IT UP TO HIM.