Accountants & Bookkeepers aren't charged, the model is to charge the end customer (business owner). The accountant or bookkeeper can choose to purchase Xero and then charge their client for it, or just absorb it into their existing pricing.
Through the Xero partner program there are some perks, including software to manage their practice.
http://www.xero.com/au/partners/
No, only clients that want it. Accountants and bookkeepers can have clients on Xero as well as other platforms. There are incentives to have them all on Xero, but there are no requirements to be 100% Xero.
Sorry, I wasn't clear: Let's say that I want to manage my clients' accounts using Xero, but my clients have no interest in using Xero themselves. Does that mean I have to pay around £19 per month per client?
Through the Xero partner program there are some perks, including software to manage their practice. http://www.xero.com/au/partners/