depends on your status, it is unlikely that the base rate will move above 2% in the next 12 months. The normally percentage the bank will add is around 2/3% above base rate for variable rates.
IF you are in a position that you can pay 5% in 2 years time it would be worth going on the variable rate and fixing it when interest rates increase.
However you can fix for 5 years at around 4.5% which gives you piece of mind, knowing what your repayments will be and when the interest rates do increase in a couple of years time you will be onside (have a better rate than current market rate)
so in conclusion
if you want secuirty fix a rate for as long as possible at a rate you can afford, ideally under 5% for 5 years, as current expectation is that interest rates will increase over the next few years as the economy recovers.
if you are happy to take a risk that they wont increase quickly to make short term gains don't fix. when i say short term i mean around 12 months.