For anyone that is interested, as I couldn't get a cashier's check, I went in person in Edinburgh today to pay by credit card. They have counter service from 9-11.30 & 1.30-2.30 on Tuesdays and Wednesdays.
I got there at 10.30 and was told it was too busy and to come back (might have been because of half term, maybe not).
I travelled up from Liverpool and was not leaving without applying, so got there at 12.55 to queue with at least five other people, all there to renew in person. It literally took two minutes. So: no problem if you live in England to go to Scotland (much cheaper and easier for me in the NW), pay by UK credit card. Added bonus: I love that the consulate is a Georgian terrace and totally walkable from station and not a fortress.
Hope that helps anyone that doesn't have the option of getting a cashier's check/money order, etc