Surname advice ! UK

So I’m not married to my partner and we’ve been together 3 years not engaged or anything I don’t want my baby to have just his name and I don’t want it to be the main one either ! You can do two surnames but mines quite long and his is short which order is best to get ? And which would be seen as main ? Or is it bad for me to get just mine I’m totally up for changing it when we get married but I don’t really even know when that will be