first of all - if it someone else and he is paying a solicitor then go with him
( at £200 / h)
Drug driving is strict(*) ( you do it, doesnt matter if you wanted to or not)
and is binary(*) ( now ) or all-or-none ( 1960s) (*)
that is, any detection is positive and may earn a summons
https://www.gov.uk/drug-driving-law
https://www.sentencingcouncil.org.uk/wp-content/uploads/Drug-driving-guidance-final.pdf
I have to say I cant get worked up with charges like driving whilst doped with heroin - whaddya expect for chrissakes
I imagine that most people on oramorph wd drive sensibly
(*) wild prolonged screams of "foo wot all dis den"