How do I get a number plate for a car that has only ever had a private registration?

My partner will be looking to sell her car in the near future and is struggling to find out how to deal with number plates. She bought her current 2012 Corsa brand new. It has only ever had a private plate...

Where can I get a cheap private plate?

How can I get a private number plate without breaking the bank?

Can I put a private plate on a leased car?

Can I put a private plate on a leased car?

How much does it cost to keep a number plate on retention?

I have owned a personal number plate for forty plus years, I have had an offer to buy the car but want to keep plate on retention. I believe this will cost £80 and then £20 a year to keep it on retention....