existing wall penetration 101:
1. bolt block to wall with through bolts
2. prop each side with acrow props
3. remove horizontal bed joint with angle grinder
4. install angle lintel
5. do vertical cuts
6. remove bricks
7. plaster to make good

heres a mid progress shot of my kids play room i did myself:



rendered up with plasterbeads


for a double cavity wall use 2 angle lintels, 1 for each leaf

you only really need a T lintel for a massive double door garage or the like