The fixed rate mortgage is especially popular among UK mortgage-holders because of the security it gives the borrower. A fixed rate is guaranteed to remain at the same level for as long as is specified in the mortgage offer, meaning that your payments will stay the same for this initial period too.
For this reason fixed rates tend to be particularly popular among first time buyers, although in more uncertain times they also prove very popular with those looking to remortgage. An important feature to look out for, with some fixed rate deals, is early repayment charge 'with overhang' : this is where the lender will continue to charge redemption penalties for a period of time after the initial fixed deal has ended.
Normally this will mean that you will be tied in to a variable rate , that no-one can predict with any accuracy. Deals which allow you to switch product, or lender, after the fixed period ends, tend to be more popular, as they don't have the same element of uncertainty.