Why would they only work for the most able though?
Because the less able (and also bright people with a poor visual memory) simply cannot cope with the memorisation (or mental imprinting) involved.
There is no logic behind most of the quirky English spellings (e.g. sEEk, spEAk, shrIEk). It's simply a matter of imprinting on your mind what looks right.
The pressures of spelling tests do nothing whatsoever for naturally poor spellers, other than giving them sleepless nights and making them hate school.
They are great for good spellers (and the majority of people on this forum probably fell into that category at school).
A simple way of enabling all children to spell better would be to make English spelling more sensible (as all other European countries have done over the last two centuries, to varying degress). But that is not going to happen (because Samuel Johnson declared 250 years ago that it must not be done).
So the number of poor spellers in the UK won't change much as result of this latest return to basics. Teachers turned away from regular testing before because it was found to be of no help at all to those who are not naturally gifted spellers.