I struggled to get an answer to this but eventually with the help of the suggestions I was able to work it out, here is the finished formula:
=IF(AND(H4="Basic",G17<450),"Basic",IF(AND(H4="Basic",G17>=450),"Standard",IF(AND(H4="Standard",G17<450),"Basic",IF(AND(H4="Standard",G17>449,G17<700),"Standard",IF(AND(H4="Standard",G17>=700),"Standard+",IF(AND(H4="Standard+",G17>699,G17<800),"Standard+",IF(AND(H4="Standard+",G17<700),"Standard",IF(AND(H4="Standard+",G17>=800),"Platinum",IF(AND(H4="Platinum",G17<800),"Standard+","Platinum")))))))))