Obviously the safest place is cash, but you will get virtually no interest on cash savings at the moment, and monthly interest accounts pay slightly lower interest than annual interest accounts. Some fixed term savings have monthly interest options. Try looking at www.moneysupermarket.com or the Savings section of www.moneysavingexpert.com.
If you're prepared to run a little risk, you can invest up to £10,200 every year in a unit trust ISA which means any interest you receive will be tax free and doesn't have to be declared on any tax form. As I'm not a financial advisor I can't give specific advice, but many people opt for popular equity income funds such as the Perpetual High Income fund or Artemis Income fund. In funds like this you can either have your dividends reinvested to buy more units, or buy income units where the dividend is paid out to your bank, although this is normally paid quarterly, rather than on a monthly basis. Such funds have an annual yield of between 4% - 5% but investing in the stock market is not as safe as cash, even though many people are going this route at the moment because of low interest rates on cash deposits.