Welcome to Flyertalk Crispin Munns, welcome to the BA forum, it's good to see you here, and yes you have come to the right place. Hopefully we will see plenty more of you too.

With a few somewhat elusive exceptions, there are very few Codeshares which are standalone on BA. As mentioned by bigjono generally you have to bolt on another BA flight so MEL-SIN-LHR for example. What may help (typically if you were 1 flight short) is to buy that flight, fly the MEL-SIN sector now-ish and the SIN-LHR sector much later. The one exception that occasionally pops up here is a standalone KUL-HKG service on Cathay but Sold As BAnnnn but the fare buckets are not cheap for that arrangement.

Inside Europe it's a lot easier. In addition to the BA and IB options, there are standalone Codeshares on Vueling and Aer Lingus Sold As BAnnnn and some are relatively inexpensive. It's not the case today, but on many days the very cheapest BA qualifying flight is AMS-LTN on Vueling.