Hi mate , your best bet IMO is to put your own small sub board next to the main board, a small 4 way board with 4 rcbo,s will cost you know more than £80 , this way you can test your own work with out having to replace the old board and test the whole house if you want to keep the cost down..( if there happy to pay , full cu swap)
Taking a 10mm up to run a sub board with shower , ring , lights and smokes on it is a definite no no , if i,m reading your post right ?
HTH
Ste
ps try
Screwfix.com - Where the Trade Buys for your boards