No. The issue is that at the moment from Cambridge to London FCC (to become part of TSGN) are the "fast" operator, with AGA the "slow". From London to Portsmouth, SWT are the "fast", Southern (to become TSGN) are the "slow". The routing keeps you on the slower tickets.
Would "via Gatwick and Bishop's Stortford" (VIA GTW & BIS) be workable? Or too confusing in its shortened form?
Showing my lack of South East network knowledge there!
Remember the new ticket formats allow for longer descriptions, so "via x and y" written in full become viable in most cases.