This as Ken has said has come up before and there is no clear agreement.
I live in France, however we are still governed by EU law which unless the UK has chosen to be 'stricter'should be the same. The leaflets that come with my sharps box say that cannulas, tubing (from pumps), testing strips, (something a lot of people throw in the household waste) needles should all be placed in the sharps box: no mention of disposable pens or vials. Certainly when in hospital pens and empty vials go in the normal rather than clinical waste bin.
The info from the Novo website says:
' Always consult your local regulation regarding the available options for prefilled device disposal. In general, the insulin pen can be disposed of as household waste provided that the needle has been removed. The used insulin pen can then be incinerated releasing only CO2 and water. Land-filling of a disposed insulin pen will not harm the environment beyond the actual use of land, because no chemicals will be released.'
http://www.novonordisk.com/diabetes/insulin_pen.asp
You may be lucky and find clear local instructions, those that I have looked up have don't mention how to dispose of pens; indeed some local authorities in the UK are still saying it is OK to put sharps in a sealed container and dispose of them with the household waste. (definitely against EU regulations)
Maybe someone needs to ask Diabetes UK or their local environmental waste authority for a definitive answer.