For example, if you are borrowing £200k over 25yrs, a 5yr fix is around 4%
£200k, 25yrs, 5yr fix
4% = £1056
To fix for 2 years then go out to tender again for the remainder:
£200k, 25yrs, 2yr fix
4.5% = £1112
After 2yrs your balance will be around £190k
190k, 23yrs, fix depending on what you chose:
5% - £1160
4% - £1054
3% - 954
So for the first 2yrs you’d save £56 a month on a fixed 5yr, then if rates stayed at 4% you would be paying roughly the same so even whatever you do (if they went up you’d be winning!)
If you fixed for 2yrs you would overpay £56 for 2yrs, then if they went down to 3%, you would be saving £102 a month for 2yrs so overall saving £552 over the 4yr period, but then you would probably have to pay £1000 to switch your mortgage, so you’d be down £448 overall and have to pay again to move it.
Unless you want to move in the next 5yrs I think you’re better off fixing (or unless you have an extremely large mortgage and think interest rates will fall drastically then you might want to take the chance and only fix for 2yrs).