I would defenitely ask more than 50 pounds for Copper Glow since she is complete. And she is still hard to find.
As for Petal Blossom, maybe 20 pounds. It's a common pony.

You are mentioning the past 3 years. Most of the world was closed due to the pandemic. And if you can't go out to the post office then you don't sell online.
Those who did sell were able to ask a lot more money for something that was way cheaper back then. Even I noticed even though I am not on your side of the pond.
But now that it's possible to thrift and go to conventions again there might be a chance you find something you are looking for so long. And when buying from pony people the price could be better too.

Back in the days Paradise Estate was also very pricy new in box.

Any area that is NOT in the city center will do As said above a lot of people are coming to party. There are really a lot of markets, people are allowed to sell in front of their houses. I did that last year after finished thrifting. Maybe by that time check out the local map for the activities because it will be listed.
I have to mention the weather If it rains, those markets also will be there.

It depends a bit where you are in the UK to point out the thrift shops and car boot markets if its G3 you are after

I need that plush when it's released

Gosh, these persons obviously don't think about the "waste" a child can bring to the climate. Just being sarcastic but you get my point

Never heard comments about keeping toys away from kids. The main thing where I live is to donate toys for the holidays. There is currently a commercial that shows an attic full of 2nd hand toys. And the kids are asked if they are ok to receive 2nd hand toys. So that's also sends a message not to waste. Nothing to feel guilty about too.