Wikipedia states "The average purity of street heroin in the UK varies between 30% and 50% and heroin that has been seized at the border has purity levels between 40% and 60%" (https://en.wikipedia.org/wiki/Heroin#Adverse_effects). It doesn't actually have a citation that confirms that, but another quick google gives http://drugscope.org.uk/how-pure-are-street-drugs/ saying "The 2010/11 police recorded average purity for heroin was 30 per cent."

So I'm inclined to believe the 30% number unless you have a source for thinking it's much lower.