There are currently 751 MEPs of which just 73 are elected by EU voters. This number will remain unchanged if the UK participates in the elections (though if/when we leave 27 of ours will be reassigned, reducing the total to 705). Even if the entire UK contingent is minded to cause disruption (unlikely in the extreme) less than 10% of the delegates will hardly be able to cause much disruption.
Ironically it’s the Euro elections where the Brexit parties can make a mark: in a general election all they’ll do is poll enough votes to stop the Tories winning but not enough probably even to win a single seat.