Companies providing annual commercial insurance policies will demand to see proof of relevant certification (A2CofC/GVC/additional modules) before they insure. Recreational insurance also requires proof of competence (CAA issued
A1/
A3).
According to the CAA: currently there is no differentiation between commercial or recreational drone use and there won't be a change there until they re-write the old PfCO to fully incorporate all types of drone (commercial & recreational) so from that standpoint, doing a friend a one-off favour and getting yourself some valuable experience

means the flight is not commercial and a recreational policy should be okay.
But horse-sense would suggest that third party cover is still a very, very good idea... even a rec. policy - especially when you'll be flying around/over/near someone else's property.
Join the Grey Arrows drone forum and have a look at the resident insurer there. Good recreational policy, realistic cover and a very reasonable annual premium.
Regarding photographs of people or buildings: Anything that appears in a photograph you take that wasn't planned to be included as a specific subject is defined in British law as "incidental inclusion".
Buildings that happen to be next door to a house you shoot are incidental to the main focus of the shot.
If people are standing in the area or walking through the frame as you take a shot: their presence is not planned beforehand (incidental) and you didn't say "...can I take your picture...", so you don't have to secure permission to publish the image. The rules of street photography apply.