English high and mid non-central vowel phonemes fall into two categories:
- tense /i e o u/
- lax /ɪ ɛ ɔ ʊ/
Beside the tense/lax phonetic distinction (which refers to the muscles at the root of the tongue), these phonemes are distinguished in several other ways -- language loves redundancy and builds it into the structure whenever possible.
For instance, all the tense vowel phonemes are diphthongized, whereas the lax vowels are pure vowels, which may be neutralized with their tense counterpart in some environments (Mary, merry, marry), but never appear with an offglide except in local variants.
In the high front vowels /i, u/ the diphthongization isn't always distinctive because the tongue gesture is so short, but it's there and shows up in transitional /y/s and /w/s in phrases like be able or do it. It's clearer in the mid vowels, where the distance the tongue has to move going from [e] to [i] or [o] to [u] is longer and more easily distinguished.
Indeed, pronunciation of Spanish /e/ as /ei/ and /o/ as /ou/ is one of the characteristics of an English accent. English speakers are normally unable to distinguish the two in Spanish, just as Spanish speakers are normally unable to distinguish English tense vowels from lax, leading to ship sheep late let Paul pole foot boot vocabulary problems.
There are complexities. In RP, as noted, the actual diphthong is centralized to [əʊ], and the Northern Cities Chain Shift has screwed up urban American English vowels almost as thoroughly as what the Great Vowel Shift did to Middle English vowels.