Hi Geog failure
Nightmare bit of homework, and it seems to be a standard part of the curriculum for Year 7. However, when we did it last year we were given a random part of an OS map which had clear contour lines.
I tried Google Earth for you, but there doesn't seem to be any source of maps that just show contour lines.
You could try getting the local copy shop to blow up the relevant section of your OS map - that is recommended on the OS website (education section), so there is no issue with copyright.
Otherwise, just do his grandparents home area or the nearest green space to you, and explain to the teacher - send in a photocopy of the local OS map with your note!! I don't think the teacher has thought this through - s/he is just blindly following a standard lesson plan.
BTW, if you actually have to build a model, cut up supermarket cardboard boxes to create the different contour levels.