Every country, city, and government has a bias and an attitude toward drugs, including London. I hope you're not suggesting (perhaps you improperly worded your comment) that the US government is biased toward drugs but English government is not biased at all and is completely neutral scientifically. If that's what you meant, I think it's specious.
Every person has a bias towards or against anything they choose to take a position on.
I'm saying that the biases of the U.S. government, UK NIH, King's College London, and Palermo University, Italy (the other sponsor of the researchers) are likely to be largely uncorrelated when it comes to drugs. The U.K. does not have the U.S's history of criminalizing specific drugs (marijuana, heroin) simply because they are used by marginalized groups while legalizing others (nicotine, alcohol, caffeine) because they have powerful corporate interests backing them. They undoubtedly have other biases, but in a study of the genetic effects of THC on psychosis those are less likely to be relevant.
One way to minimize the effects of bias, despite its ubiquity, is to seek out multiple independent POVs. Each one of them is individually biased, but when you average them together you get something approximating the truth.
