In this situation specifically, the win rates support UKF being underpowered, but given the context, small sample sizes, and overall uncertainty, there are A LOT of other arguments it can also support, which is why I disagree with drawing any single conclusion.
Because players aren't a homogeneous group we'll never have a particularly robust sample for any faction, nevermind the least popular one. You work with what you get.
I don't think there's any reasonable conclusions to draw from what we have other than "Brits are worse against both axis factions than soviets are".
Beyond that, all of the axis wins that you are referring to are ostheer wins. This begs the question of whether it was UKF that pulled these players into losses, or ostheer that pulled them into wins (or neither).
Ost performance in the tourney is much less abnormal than the brit one, making this sort of claim while waving off the data points we have for brits is doing the exact opposite of what you've been saying about the sample size etc.