I have a few friends who live in the UK, who I send mail to.
1st class stamps tend to be used when you want your post to be sent quicker, whereas 2nd tend to be sent later.
If you are send a rather large package, and can't go to the post office, you'll need to put a couple of stamps on the package, as the stamps will pay for the weight of the package. However you can also go to the post office, where you pay around £0.50p - £6.00 depending on the size and weight, and they'll put the correct stamp on for you.
Royal mail is the delivery service in the UK for letters and small parcels. Larger parcels are sent via parcelforce. The post office is where you go to buy stamps/pay telephone bills/send overseas packages etc.
No difference between royal mail and post offices. Royal Mail is the company that runs the postal service, and post offices are where you go to send letters/weigh packages before you send them and things like that.
First class are supposed to arrive one working day after you send it, dependent on where it is sent from- but this rarely happens in my experience.
Second class can take 3-4 working days to arrive, but it's cheaper.
Love that Boris Johnson you have flying around there haha!
First class and second class are the same as what mentioned and there isn't any difference between the Post Office and the Royal Mail Royal Mail are just the company that own the post offices hope that helps