....which lets me depart from Victoria but allows me to return to Clapham
Junction.

Personally I would have thought that a normal return ticket from
Victoria should be enough as, I thought, breaks of journey are allowed.
But I tried this before, the Clapham Junction barriers rejected me, and
the person at the gate let me through but told me it was not allowed and
not to do it again.

As I will be doing this journey again shortly, just using a cheap day
return or whatever they are called these days, is there an official or
hassle free way of doing this?

Thanks.