Flying abroad for the first time as an adult - advice please 🙏 UK

Im flying solo from UK to Finland at the beginning of July and want to be as prepared as possible as I have no idea what I'm doing.

Some things in particular that I'm clueless about:

.GHIC NHS card. Is this something I should apply for?

.Travel insurance. How do I know who to go with and what kind to get, if there are even different types? (They offered it when I booked my flights but Google said they generally hike the price if you buy with the airline and to get it seperately.)

.Can I use vaccum sealed bags in my luggage as I'm only taking a carry on for a 10 day trip? (Flying with Ryanair)

. When do I check in for my flight?

. I have a prescription medication. Do I need special forms or something to bring this with me?

.How early should I be at the airport flying from Stanstead at around midday mid week?

Any other advice/tips appreciated 🙏