This depends on the requirement for carrying 5 adults in comfort. Myself I'd say most medium size cars (eg Golf) carry 4 in comfort but the 5th person sitting in the middle seat would not be comfortable on a run.

With a 4k budget then you need to be careful - avoid anything complicated (eg avoid anything with a turbo or supercharger) and CONDITION is all. Thoroughly check service and MOT history https://www.gov.uk/check-mot-history and do a HPI check! (Look online - this is not too expensive these days). Also watch out for the starting of rust on any car - this problem has not totally gone away.

As SLO mentioned above somehting like a Civic 1.8 is a great car although check the clutch is good.

Another one that fits in the budget that I like is the special edition of the Octavia produced in 2011 (1.6 SE MPI) that has a reasonable spec and is fitted with the older design 8 vlave petrol engine. This is a very sturdy engine and is capable of doing high mileages. Check the air con works and the abs light works (iluuinates for a while and then goes out). This has a huge boot as well.

Indeed - essentially the petrol engine must be Euro4 (or higher) compliant, as my MY05 Mazda3's 1.6 petrol does. I suspect the others mentioned do (the 1.8 Honda defnitely will be), but perhaps not all of the Fords, though perhaps only in the early mk1 Focuses. Any petrol engined car under 10yo will be compliant and thus be allowed into ULEZs for at least the next 5 years, maybe even 10 without penalty.