We had a broker recommended to us when we set up our business. My theory was that I couldn't be away from the business for any unnecessary length of time so wanted private medical due to a poor NHS service in the local area and a desire/need to, if the worst happened, to be treated quickly.
Generally, a broker would have access to all the available companies and could recommend a provider easier for your needs as he/she would understand which providers cover things like pre-existing conditions etc. However, as experienced in the mortgage industry, some brokers may point you towards a provider who will ultimately fit your needs BUT gives him/her a better commission rather than being the best provider.
So to be honest, a good broker will save you a lot of time and phone calls but you take your chances when finding one that will look after your interests first and his/her last.