When we knocked through a wall, we spoke to building control who gave us the telephone numbers of a couple of structural engineers who worked out what size we wanted. We got a RSJ locally and had our builder pick it all up.
Whilst it did cost a couple of hundred quid for the engineer, we knew it was done correctly, and if the wall were to fall down, we would have a comeback.
Also, if you don't contact building control, you will come into all sorts of problems when you come to sell your house.