With the summer holidays coming up, getting the kids into making kits could keep them occupied. This is good starter kit. £6.12 @ Argos.
Will need glue and (optionally) paint as well. Can be ordered with the kit from Duncan's but not available from Argos.
Note: There is free delivery on this item - don't be put off by the banner at the top of their page that says free delivery on orders over £10. Check the delivery cost for the item and it shows free delivery on orders over £5. Also logs as free delivery on Google.
More detail (from the Argos site!):
Plastic scale model kit.
Skill level: 1 (where 1 is for the beginner and 4 is for the more experienced modeller).
Number of parts: 38.
Construction and Painting required: Glue and paints need to be purchased separately.
Suitable for ages 8 years and over.