This is a 'change of airport (in London) and/or airlines; kind of deal but the good news is, there's a long layover in London so two trips in one!
Virgin Atlantic / British Airways, via Priceline have round trip airfares travelling from San Francisco (SFO) to Pisa Italy (PSA) for $478, making 1 stop in London. I have two sample itineraries below.
Departs SFO: Sept 25, 27; Oct 2, 4-5, 8-10, 12-16; Dec 14-17
Returns: Oct 2-5, 7, 9-12, 15-18, 20-23; Nov 6-10, 13-18, 20-25, 27-30; Dec 21, 23-24, 26
Use dates and book it on Priceline [priceline.com]
. Bundling hotel with airfare will save money.
Pisa Travel Guide [italyguides.it]