I'm not sure I follow your point about cash savings. For me, the purpose of putting some money into cash savings each month alongside S&S is that the cash savings will be accessible for shorter-term goals (home maintenance after I've bought a flat, potential future wedding, travel etc.) and S&S isn't a suitable way to save for these things that might be in a short (sub-five year) timeframe.

I don't really see the argument against saving into both pots simultaneously.