You are over simplifying it. Only the change to 15 mph in 1983 was to get closer to the EU's 25 kph. Nothing else was, we kept to our 200 watts rather than the EU's 250 watts, and what's more we also followed by introducing a 14 years lower age limit that no other EU country has.It seems to me that it was ok for years as you've stated below that from 1983 to 2003 15mph was legal. 20 years is clearly many years. So, the use case was simply to fall in line with the EU? Now we are out of the EU, what's to stop the law on throttles being changed?
And finally on the legal position, we have never at any time said that throttles were legal. Our poorly drafted law simply omitted to say how power was to be applied, the legislators probably assuming we'd do the same as the EU.
There is nothing to stop us changing the law now, but it won't be. That's because the government has made it clear that they want our law to stay aligned to the EU's. To facilitate that the Great Repeal bill was passed before we left the EU, which wrote all the previously accepted EU law from 44 years into British law.
And the government position on keeping EU alignment has been reinforced by them saying that if they do accept powered scooters into law, they want the EU's 20 kph (12.5 mph) limit for them, not the USA's 15 mph.
Yours is the latest of many attempts to get these changes, there's even been a petition to parliament, but I say to you as I have to all others, you are banging your head against a brick wall. It isn't going to happen so just be content with what we've got. Remember, it could just as easily be made worse.
.