Place on the outside of the top of the cage. Make sure the top is mesh though and not glass Uvb cant pass thru glass. Check the temps also you dont want the cham to get a burn if the bulb is too close to his/her basking branch.
that seems to be more common in the UK. The majority of us put the bulbs on top of the enclosure but then again most of us use screen where you guys tend to use the glass and wood enclosures from what I have seen.