In practical terms what you want to do (pay in a large amount of change) isn't a frequent requirement for the bank. Perhaps 1 in every 200 people who visit a branch (at best) will want to do it. The bank has to focus on providing the best service possible at reasonable cost to it's stakeholders and given the low demand, providing the ability for you to pay your change in on a saturday isn't worth the cost (which after all is represented as a lower interest rate on your deposit or a higher rate on your overdraft). However they do provide still provide you the ability to do it, at slight inconvenience, which is really the best you can expect.
That isn't what annoyed me though. Up until recently I could use by local branch on a Saturday, but now I can't. So, I made the effort to find another branch which could help. According to HSBCs website, the Richmond branch DOES have a counter service on a Saturday - hence why I went there. I made the effort to check this before I went, they couldn't be bothered to make sure the website info was correct.