You can't have both the safest bet and the best returns. You need to decide which is your priority. A cash ISA should certainly be part of your plans and, if you are comfortable with the risks, a stocks and shares ISA could bring the total invested up to £7200 and you could put the rest into premium bonds or a savings account.
When considering safety, large UK high street banks and building societies are always going to be safer than someone who you have not heard of before and who doesn't have a branch near you. I do not know who is topping the savings accounts rates charts at the moment but I do know that the position was recently held by Icesave....
You should only deal in large amounts with an organisation that you know and trust.