Theoretically speaking, this is not theft. You are correct in that theft requires specific intent: to permanently deprive someone from the use and enjoyment of one of their lawful possessions. In real life, it is a matter of evidence of course. Saying you were only borrowing can be proven otherwise based on your actions. I should also mention that if you "borrow" something an there were obligations attached to that borroeing (e.g. "you can borrow it but you mustnt use it like this, or you must return it in a week") then failing to abide by those obligations can be sufficient to show intention to deprive. Furthermore "borrowing something" and consuming its entire usefulness for yourself counts as the permanent deprivation of the use of the possession from the owner. E.g. "borrowing" concert tickets and returning them after a concert (see s.6 Theft Act 1968) R v Lloyd, Bhuee & Ali  QB 829 also tells us that the condition of something borrowed once returned may be indicative of whether there was an intention to permanently deprive someone of their rights to property.