As far as I am aware nothing has changed on the visa front when issuing the 33 months first entry visa.
The 3 months were initially given so that the person leaving Thailand (or elsewhere) had time to wind up their affairs and still be able to get 30 months residency in the UK.
If you left LOS early and entered the UK anytime during those 3 months it was a bonus, then when it came to the next visa the requirement was that as long as you had been in the UK for 30 months you could apply for the next visa, BUT you could do this 28 days prior to the 30 months anniversary of landing in the UK.
Not sure about the English test but from what i have seen previously on here as you said, as long as it was initially used for a previous visa it was still valid. Someone will need to confirm this though.