Yeah, but that is the point - the quotas necessary to maintain said ecosystem should be the same as those given by the EU. If the data they are working on is correct, the sentiment you are talking about is simply borne out of shortsightedness and describes the tragedy of the commons. That is because people do not take the future sufficiently into account when they determine whether or not more was taken than given. People simply tend to value 'being able to fish more during the next decades' higher than 'not having less overall fish to fish after said decades.
In the end, letting the UK unilaterally fish more just means they make more at the expense of either future generations or fishing villages in other countries sharing the same resource. If they want to fish more in a fair way, they will have to build a wall around "their waters". Until they do, it is not "their" waters in the slightest.