There was a change last year in this area where by the whole of the Worthing CTA now is subject to a transition altitude of 6000'.
I believe the rules are that it is permissible to cross an airway at its base level at right angles, I'm not sure that is possible with the mix of airways in that area all going in different directions. If that is true, then you would have to remain 500' clear, which with a QNH of 1000 would mean 4'600ft maximum give or take.