I got a really cheap fare in a flash sale and treated myself to a cabin bag (the rucksack under the seat was free).
At the airport (LGW) everyone with any sort of luggage, has to check in, so the queues are long and slow.
Don’t roll up thinking you can just walk to the gate.
You check in with hand luggage, so that they can weigh it and then charge you more when you’re over.
There were a lot of people emptying hand luggage cases at the side of the check-in desks and putting all sorts in their pockets and handbags etc.
Once on board, if you can resist $$ for a microwaved meal for one and you can survive 8 hours on the snacks you purchased at the airport instead, you’ll be fine.