I've booked an inbound flight from London to Lisbon, Portugal but not an outbound flight! What do I do?

I've never travelled abroad by myself before and I decided to use Skyscanner to book my flights.

I'm from London and would like to holiday in Portugal for a week.

I booked a flight from London to Lisbon with Ryanair on skyscanner but I had to book a flight from Lisbon to London with Wizzair. When I try to book the ticket it kept making me accept all this rubbish I didn't need. I accidentally closed the window on my phone and I'm also private browsing.

Now I've got a flight booked from London to Lisbon but not the other way around!

What do I do?

Please help me!

Added (1). wouldn't it cost a lot

London 8:30 hour layover?

In a few days i will have a 8:30 hour layover in london where i need to change airports.
I arrive at Heathrow Airport at 7:40 and leave Stansted airport at 16:15.
We want to visit one waterstones and have lunch at an Honest burgers.
Is this possible? How can we do this in terms of public transportation? Is there another restaurant that is not super expensive that serves the impossible burger? (We want to try impossible/beyond burger)

Why don't airports use steam catapults and transverse arrestor cables, like they do on aircraft carriers, for extra short take-off & landing?

1. Airports could be pretty tiny with runways no longer than a football field
2. Takeoff and landing (takeoff especially) would be *much* more exciting
3. Airlines could charge people more money because the experience would be much more fun
4. It would save a lot of fuel with much less taxiing around the airfield, and much less fuel used during the takeoff phase