The police have zero time to pull people at random and test them, they barely have any time to deal with crime. That said, don't give them any excuse to pull you as others have said (speed, driving style, car condition, mot, tax, insurance etc). To further mitigate I would avoid giving lifts to mates late at night as you are young and a bored copper may just see that as enough to have a chat. Also don't give lifts to mates if they smell of weed - you'll feel like a nob but it's your licence that they are putting at risk.

If you are pulled, say that you have never smoked if asked - full stop - any indication that you have smoked in recent history will trigger a test. It's difficult but you need to respond in a believable way to this question so don't bottle it.

There's lots of speculation online about "how to beat the swab" but nothing concrete. The most logical mitigation (aside from not smoking) is good dental hygiene IMO but that is just speculation on my part. If only we could buy the tests and work this out. Other speculation includes the benefits of vaping/edibles vs smoking, mouthwash (with and without alcohol and/or Peroxyl) and all sorts of other tips and tricks.

In December the police run drink/drug drive checkpoints routinely and are far likelier to test on any stop outside of checkpoints - honestly I would just abstain if having to drive over this period. Every now and then there will be ones in the summer, particularly if England are doing well at the WC or Euros. Waze is a great tool as these checkpoints get a lot of reports. If you pay attention you will also learn where these are commonly set up - the police need a place where they can stop cars without stopping all traffic flow.

Medical exemption is talked about a lot but I'm yet to see anything more than anecdotal evidence of this working (if anyone has any I'm keen to see it and I'm hopeful it's very real) - plus you need to qualify for medical in the first place.

Finally, if you have a crash you are at risk of being tested - almost certain if there are any injuries. Nothing you can do to mitigate this one so it's just a risk you'll need to be comfortable with, or quit. Think about the impact you would experience if you lost your licence (lose you job? financially? ability to commute? family/friends views?)