If I understand the small print, provided you borrow a minimum figure of £5000 of Honda supplied finance, the car receives the 5 year service plan gratis.
I'd like to see the numbers, none of the Honda finance examples relate to modest loans of £5000, but if you are a cash buyer but elect for a £5000 of Honda finance over a year, the interest over the loan period is probably about £100 and you "win" £650 minus £100.