I just tried to renew a license that's due to expire on the 22nd of November. I can't for the life of me find a button that allows me to do this. All I can do is deposit account credit. Isn't there a way to renew a license in advance for 6 months, or a year?

You're a bit ahead of the client area. I seem to recall notifications of pending renewal are sent out 7 days prior to expiration and the renewal invoices generated 3 days prior to expiration. So you won't be able to renew until 3 days prior to expiration. If you have a credit card on file, or account credit, it will automatically be charged for the renewal when the invoice is generated.

The only method currently available to pay up-front would be to add sufficient funds to your account credit. When renewal invoices are generated, they would pull from your account credit before charging your credit card or PayPal account. So simply deposit the required funds to your account credit to cover two renewal cycles and you've effectively renewed your license for a year.