Northern Ireland, Scotland and Wales are devolved government. greater London is NOT regional government, nor is it devolved government, that is why Boris is not “first minister” but “mayor” and elected separately.
The GLA is smaller than the GLC in numbers because it is a toothless assembly, the power rests with the mayor. Greater London has no devolved powers so it is wrong for you to say it is the only “region in England to have devolved powers”.
Compared to Scotland, Wales and Northern Ireland, Greater London has rather limited powers in terms of devolved powers from Westminster
However Labours plan (in the late 1990s) was for the Regions of England to also receive devolved powers, however this plan was unpopular and was scrapped after the region of North East England rejected the idea
For more info about the Regions of England, its on this webpage:
Regions of England - Wikipedia, the free encyclopedia
However, Greater London (which is classed as a region of England) did vote in favour of a regional assembly and a mayor (the only reason he is a elected is because the role has more powers than a “First Minister” and hence can be counted as a devolved region of the UK, although it is of a limited kind compared to other devolved regions of the UK
However I feel it should gain more powers (up to a point where has similar devolved powers to what Scotland has), establish its own symbols, formally separate from England and become the “5th home nations” of the UK and annex the rest of the metropolitan area and commuter belt of London
Also when I meant the GLA is smaller than the GLC, I was not talking about the size of the assembly, rather the number of employees (or bureaucrats as they are known) as well as the powers and responsibilities (the GLA has less of all 3 than the GLC)