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My interpretation would be that some growth is assumed/forecast, e.g. 1%, and therefore "-7.7% relative to growth" means -6.7% in real terms.
You would report it like that because if growth was 8.7% and Brexit caused -7.7% then in sum you get 1% increase, so on the surface it would appear that Brexit didn't really do any damage, which obscures the fact that without Brexit we would have had an incredible year.
But that's 100% total guess/speculation.
When the governments paper on Brexit impacts say "-7.7% relative to growth" what does that mean?
A reduction in the estimated growth figure of 7.7%? Which would be -7.7% of 0.3% at the moment, for e.g.