If you live in UK, you should have no problem ordering from Germany. The mail order houses there sell mostly lights that are street legal in their home country, and label non-compliant lights as off road only.
EDIT: The above goes for bike-mounted lights. Helmet-mounted lights do not seem to be under the same regulation.
If you absolutely must buy from UK, you can look up models on German sites and see if they are available in UK. Although, UK and continental Europe seem to be strangely separate markets even if they are in the same customs union.