Yes, the break fee is calculated based in movement in funding rates.
Moderator's note: for anybody who is thinking of switching their mortgage (or re-fixing with their current lender), consider posting your mortgage details in the switcher thread (in the format shown in the first post). Someone will calculate an estimate of the break fee (if any) and of the...
www.askaboutmoney.com
Interbank rates have dropped significant since the start of the year.
What's be interesting to see in your case is if they're calculating based on 2 or 3 years (I think they made a mistake in your letter and offered you a 2 year rate for 3 years?).