thing is, you can have the same information as your bank.
When you fix, you are not ‘betting against your bank’. Your bank doesn’t care, they’ll sell the mortgage off, for the length of the deal, at a margin above the current swap rate.
below shows the current swap rates (ie the interest payable on borrowing money bank-to-bank) :
http://www.mortgagesforbusiness.co.uk/research-data/money-markets/
libor (overnight) is ~0.3%
2 year is 0.4%
5 year is 0.47%
10 year is 0.7%
basically this is showing that the money markets are pricing an average bank rate of just under 0.7% over the next 10 years. The additional money you pay on your fixed rate is the risk of you yourself defaulting, plus the costs to run the bank/profit margin. The closer your fix is to the swap rate, the better the deal you are getting