You will get hammered by the ERCs, but with a few caveats:

  1. Often if you move house they will let you transfer your mortgage (actually take out a similar product and avoid the refi costs).
  2. If you win the lottery, or otherwise are in a situation to prepay your whole mortgage? Well you might care much less about the 5% early fees in that case.
  3. You can prepay usually 10% a year, at their discretion, without any early fees. If you just become a fair bit richer than you expected, prepaying at this level will reduce the mortgage quite quickly.