Transit visa .spain -uk-thailand?

we are traveling from spain via the uk to thailand,we will be changing airports and be lesss than 24 hours in the uk on both outbound and return flights,my wife is a thai national with a spanish resident permit.from what i understand as a visa national who has a confirmed onward flight she does not need a transit visa, but the uk gov website says it is up to the discretion of the immigration officer which seems a little ambiguous to say the least ,any thoughts ?ps i am uk citizen