I would have stayed in Hannah and Frank's apartment building
1. It was eventually going to rain.
2. The building was relatively secure.
3. They could have improved the shopping cart fortification.
4. Frank obviously had the ability to keep their individual apartment safe as proven by Selena and Jim's hectic arrival.
5. They could go apartment by apartment throughout the rest of the building to restock their supplies.
6. Travelling to Manchester from London is a 200 mile, three and a half hour drive at best during which a myriad of problems crop up, including the death of Frank.
7. You don't know for sure what is in Manchester waiting for you.