Can I book an outward bound with one airline and return with another?

[golondrina]

I should have put this in my datchet thread. Can I fly into Stansted (from Spain) with Ryanair and out of Gatwick with BA a few days later? Just have 2 singles with different airlines?

Yes, no reason why not.

Yes, I've done it a couple if times for cheaper fares.
Just remember that baggage allowances and online check in times may differ.

Yes, most flights are singles these days (BA may be an exception, but certainly all flights sold by Ryanair etc are singles).

A couple of years ago, I flew from Leeds to Malaga and then returned a few days later from Gibraltar to Manchester on a different airline with no issues whatsoever as it was the cheapest, or best flight times probably.

Yes of course

Of course you can! I've done it loads
to get most convenient times and best prices. Only possible negative could be if it's a short trip and outbound flight is covered you wouldn't get the return refunded.

Skyscanner is good for this.

Yes, I've done this for work trips.
I've also flown one way and caught the Eurostar home.

Yes, we did that s few weeks ago.