The problem is though a fiancee visa is granted on the basis that the marriage is going to happen in the UK. If you tell the ECO that you don't plan to marry in the UK then they might not grant the visa. Would never recommend telling lies to the ECO but in this case might get away with claiming that the marriage would happen in the UK (then you changed your mind later...).
Though as the aim is to sample life in the UK prior to getting married a visitor visa is the best option, but as mentioned many times there is no guarantees that it will be granted.