Thanks for your efforts to clarify this situation. I did actually read the first email in my spam folders, but was confused by it and forgot about it.
At the moment, I think I have three Pro accounts (for my Mum, my Dad, and I) and a Business account (with five slots for my employees).
I think it makes sense for me to upgrade the Pro accounts to Prime, as they are single user accounts with static email addresses.
I am not sure about the Business account. We change employees from time to time, as any business does, so if I upgrade this account to Prime, am I paying for five specific email addresses to be upgraded to Prime (some of which might be wasted when we change staff and delete their old G-Suite accounts), or just paying for five slots which can be reassigned to new people (as is currently the case with the Business account)?
Also, how come the GBP price is the same as USD and EUR? Presumably I can just pay in the cheapest of the three for me? 
Thanks,
Tom