As a UK resident - an SAK is fine as EDC, as is any sub 3" non locking folder. Other blades can be carried with good reason, and discretion and context help a lot - a mora packed in a rucksack while hiking is probably OK, a locking Leatherman in a crowded city pub at 11pm, probably not.

It is a stupid law, but it IS the law and for that reason I find an SAK, with it's ubiquitous red plastic scales and non-threatening appearance is a good choice.

Regarding carry-on - food and water are a good idea, the right clothes help. I'd also suggest the two principal tools for urban survival are a light (e.g. a photon) and a whistle. Gets you found and guides you out. Some OTC meds (painkillers, anti-spasmodic, anti-histamine) are also pretty handy.