I don't mean to be rude or overly-critical, but... there are a number of important sounds here that are either missing or that I really don't understand the logic behind.
For one thing, there's no [A] vowel ( or a similar equivalent such as [Q] or even just an [a] ), which is important for a lot of variants of British English, in order to distinguish words like cot-caught, bot-bought, tot-taught, etc. Most British English speakers don't have the cot-caught merger the way that Americans do, so it's often an important distinction, especially considering there's not really any reliable way to make r-colored [A] sounds either ( such as in "hard" ), since now there just isn't an [A] sound, rhotic or otherwise. You essentially run into the issue of "hard" and "hud" sounding the same ( assuming [V] is what's meant to be used in place of [A] ).
There's no [ U ] either, which is liable to throw the pronunciation of words like "could" "foot", or "pull" off.
I also... don't get how an [u:] is supposed to work here. I'm guessing that [Pu] is meant to be the equivalent to that sound - but I've looked at the tutorial, which suggests that diphthongs are supposed to transition from CV's containing pure vowels, but there aren't any pure vowels that [Pu] can reliably transition from. There's the same issue with [U@] too, there's not really a good vowel for it to transition from. I guess you could try fiddling with [@] maybe, but that's pretty heavily dependent on the individual speaker, especially since it's going to have to do double-duty for regular [@] and an r-colored [@r] ( such as for Vowel-Vowels ). Certain variants of British English also likely won't allow [V] and [aI] to transition into each-other very well either, since a lot of them either pronounce [V] as more raised, slightly rounded, or both.
And [e] is confusingly its own pure vowel while [E] is also present? Even though in English they're pretty much completely interchangeable, and [e] only makes sense separated from [E] if it's meant purely for [eI] to transition from, which makes some of the previously mentioned ommissions all the more perplexing. In the pronunciation guide, you also describe [E] as "(Pronounced as the ‘a’ in ‘babj’)" - did you mean "baby" here? If so, [e] and [E] should be switched around, since [e] would be the more accurate SAMPA notation.
I notice the description for [O] is " (Pronounced as the ‘ou’ in ‘tour’)" - but most British Accents pronounce that with the [U@] vowel, along with a lot of other similar words that get turned to [O@] in American English. Perhaps there are some variants of British English I'm not aware of that pronounce it closer to the American way, but Received Pronunciation at least, usually does not. I feel a less divisive description would be something like " (Pronounced as the ‘o’ in ‘tore’)". Almost the same without the possibility of confusion that "tour" might cause.
And I'm... deeply confused by [@]'s description. Nevermind that "the ‘o’ in ‘to do’, or the ‘a’ in ‘a lot’", are two completely different sounds, the former example isn't even correct. [@] is a schwa - in English, it's typically an allophone for [V] or [ I ] when those sounds are very short and unstressed ( like in your "a lot" example ), and more relevant to British English, is the standard British pronunciation for the "er" sound, like in "bird". It's not the same as, or even really sound much like, the "o" in "do", unless you want "bird" coming out like "bood", or "boot" coming out like "bert".
[Ii] and [Pu] also just seem kind of superfluous, since they're just allophones for [i:] and [u:] respectively, and also generally pretty easy to imitate with Vowel-Vowels if you really need those specific pronunciations. I can't help but feel that just makes them confusing to approach for newcomers. People in the vsynth community already pretty largely dislike SAMPA ( for reasons I honestly don't really get ), but adding a random [P] to [Pu] in there sounds like a strange, even more alienating choice, since SAMPA already uses that sound ( since [P] is kind of like a [v]/[w] hybrid, with a weak [v] sound, and it's also not syllabic ). If you really need it, [Uu] would be more correct, or if you have to get real weird with it, [1u], [7u], [Mu], or [}u] would all also be debateably roughly correct, or at least similar.
