Why do Brits pronounce "leutenant" with an imaginary "f" in it, like "left-tenant"?

It's quite possible that it derives from the fact that the letters 'u' and 'v' were once the same letter pronounced 'yu'. They were written differently depending on their place in the word (initial, or other) and before that the letter was always written like a 'v' in medieval Latin. So the sound of the v, very similar to f, probably came from this and stayed with it.

Also, it's 'lieutenant'.

Not leutenant but lieutenant.
Changing a U sound into a F before another consonant is also a characteristic of modern Greek, where the first people to make this distortion were regarded very much as sub-standard speakers than as members of the élite. The pronunciation "Leftenant" dates from the 14th century, whereas the US "lootenant" results from immigrants learning spelling pronunciation from the dogmatism of their school teachers. Among the US military, "leftenant" persisted as the normal pronunciation until the 1890s. Spanish and Portuguese avoid the problem but dropping the first syllable and saying "tenente." I prefer the German Leutnant myself. The word's origin, like most European military terminology is French: "lieu-tenant" (taking the place of [a captain, colonel, full general, or governor]).

I suppose for the same reason North Americans say "kernel" for "colonel".

Everyone says "kernal"!

Until a few hundred years ago only a minority could read or write.At that time spelling did not make any difference for most people.English spelling is often not phonetic even according to English spelling rules.

lieutenant, not "leutenant"

See Maurice Dease- Wikipedia