I wonder whether Labour will still have all that party machinery come the next General Election? We have heard of their members voting for other parties in the Euro elections and I'm aware that a relative of mine who's been one for 50 years doing so. She was an active member too, leafleting and canvassing, but won't be in future.Labour have a fantastic local mechanism for campaigning. That is what will have win this by election. Brexit party with a supporting organisation simialr to a "normal" party might well have succeeded. At present they rely on the nige factor. Couple that with the dull party mechanisms seen elsewhere and they will be a formidable opposition ( until they nigel moves on to his next host)
Your conparison with the euro elections is not a good one. That was focused on a very narrow issue. Further Labour did not really campaign and absolutely did not deploy that party machinery to turn out the vote. They did for this by election.