Ive just had the same problem with my extension self build.
Laid the foundation mostly flat but one edge had a 25mm rise along it and also the level of it was out of line with the bricks on the house it was joining to. I spent a day or so cutting the first course of concrete blocks to take into account the foundation rise and to get everything lined up with the house on the first row. once I'd laid them every subsequent course just lined up fine and no adjustment was needed other than the slight issues with the house not being built to gauge or level!
If you're not building a skyscraper I'm sure a short section of 100mm strip on good ground will be fine, especially with 3 or 4 courses of trench blocks on top. Use sharp sand instead of building sand for thick beds.
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