There are several Illuminations handicapped viewing areas. They are marked on the
Epcot park map with a red letter I and a wheelchair symbol.
The one that was already mentioned is in front of UK on the Canada side. There is also one in front of Germany and one on each side of the entrace into World Showcase (on the side of the shop closest to Canada and on the side of the other shop closest to Mexico).
How fast they fill up depends on the night - if there are lots of people with special needs in the park that night, they will fill up quickly. They park the wheelchairs and/or
ECVs tightly together along the fence and the rest of your party will be standing behind you.
If it's very busy, they may limit the number of people allowed into the spot. We have been there when it was limited to one person allowed in with the person using a wheelchair/
ECV.
For Illuminations, there are many good spots available along the World Showcase waterway, so using the wheelchair viewing areas are not that much of an assist. Since there is a fence all along the water, there is no chance of anyone getting in front of you. Some spots also have benches nearby. If you want a very good spot, you do need to be in place about 1/2 hour ahead of time. But, you are usually able to still get a spot along the fence even in the last few minutes before the show begins. Because most people want to be close to the entrance of World Showcase to make it easier to get out, those areas are most busy. If you go further 'in' you will find more spots.