Your premium can go up as well as reduce, even if you do not claim. The reason that premium costs typically increase year on year, is that the member is due to member age increases, medical inflation is running at a much higher pace than CPI/RPI inflation and there have been Insurance Premium Tax rises year on year. This results in the average premium increasing by circa 10-15% per annum, without any claims.
There are both claims related and non-claims related policies. A non claims related insurer will assess the performance of their entire book of business and apply rate increases or decreases according to what they deem is necessary to ensure their financial stability.