Okay... Here we go:
Before anything, let's imply this is a very significant amp that they do not scale to... At all, and the Tower Feat that the Team does:
Not only states the amp Britain got under very specific circumstances, but states that the Tower connects to every plane of reality.

The next scan discusses that this wasn't just Britain and Meggan, it was the whole team of Excalibur fusing together

The Energy Matrix is causing all planes of reality to collapse, and can only be halted with
Geometric Progressio, allowing them to adjust "
Temporal Frequency" and
Rise Through the Multiverse. It is likely that these are Higher-Dimensions due to how they are described. The mentioning of the 4-D definitely helps the planes=Higher-Dimensions aspect.

"All Alternate Earths Exist in the Same Plane" is proof that the Planes =/= Alternate Earths, and the Plane referred to is likely the Fourth Dimension

They repeat this feat at the very end of issue 50 of Excalibur... Albeit context implies they were running out of power.

Proof that Feron anchored the Phoenix to the Multiverse to accomplish the "Every Plane of Reality Feat", which would require power somewhat comparable to the Phoenix Avatar in question. Also Necrom could fight Feron evenly who was empowered by the very same Phoenix who performed the feat, proving Base Nerom scales over the feat in question.

Merlyn at the time had the Energy Matrix, which was the very energy of the Multiverse itself according to statements. Despite that, he stated that he was a powerful enough opponent to avoid confrontation despite the fact he had such power.

This is it for the Captain Britain / Excalibur Segment. Feel free to analyze it at your leisure.