n the late 1980's Maplin was selling off the boards as spare parts, possibly working but without the CRT, main chip and tuner. Probably ones that were DOA from the assembly line, where they would pull the most expensive items ( Tuner, CRT and the ASIC) from them to reuse. Probably the main reason the ASIC was in a socket is that they were heat sensitive, and were the single highest cost item in the set, probably costing more than the CRT.
I watched the Andie Davies memories about Sinclair, was very entertaining on how difficult those were to make.
As to the battery packs, they were a part of instant film packs, used to drive the camera motors, focussing system and the flash circuitry. Pretty impressive for the first mass produced pouch cells, they were nice to use in projects as discards from camera packs, still having a lot of power left in them after the 120 shots from the film unit. Only issue was connecting to the aluminium foil tabs as they were not solderable. I generally used a crocodile clip or paper clip as a contact with a piece of magic tape as a cover for the clip.