Answer: I am a genius
The plan was to:
1} Take Pete's car.
2} Go round Mum's house and get Mum, along with killing Philip, who they assumed was now a zombie.
3} Go to Liz's flat and wait for the invasion to stop.
It was a good plan, but Ed didn't want to hole up at Liz's, as he didn't know where the exits were, and he wouldn't be allowed to smoke. The seond time round, it was all the same, except they'd get Liz and bring everyone back to their house. They scrapped this after realising the house wasn't safe. After asking themselves "Where's safe? Where's secure?" "Where can I smoke?" They decided to hole up in The Winchester.