If there exists a cheaper "Not London" Shelford to Tottenham Hale ticket the National Rail webpage definitely doesn't know about it, because it thinks the one it tried to sell me was the cheapest available ticket. But yes, someone on Facebook had also noted that the Tottenham Hale ticket allows more possible routes - though possibly not KX - they thought you had to change at Finsbury Park. I think that would be pretty much guaranteed to be slower than just getting the direct train though, so wouldn't be very useful (and certainly not in my case, since KX was actually the bit of London I was going to anyway rather than TH!)

From Finsbury Park or KX you'd just get the Victoria Line to Tottenham Hale - going via Liverpool Street would be a very long way round - but I guess since we've already gone a long way round via Cambridge who knows whether it's actually valid or not...