I think you can typically pay 10% off the ?principal? the principal being hte initial loan, so if you borrow 100k on a fixed you can pay an extra 10k a year off every year (but no more and you have to be care ful overpay £1 too much are you get a massive fine since you breached the terms)
Ours with RBS is 10% of the outstanding principal per year. So we can pay 18k this year, but maybe only 15k next year, 13k the year after etc.