I want to buy anew combi boiler to replace my vokera excell 80 sp which is fitted inton a cuboard so it has to?

a certain size 790x450x325 i have a 3 bedroom semi with 1 bathroom 1 shower and 2 sinks and 6 radiators which are brand new double prorad with thermostatic controls i want to chane it beacuse i dont think its the most efficiant and the gas bill is going up ..