As a protest it's better than not voting at all or voting extreme Left or Right, which is what some will be tempted to do as spoiled votes aren't counted.
Here in France many voted Front National (fascist, negationist party) to protest last time & look what a mess that made!
I can't think of a better way of gifting the seat to Labour. This form of protest will have negligible impact except to make the odd Labour politician feel a bit uncomfortable. Professional politicians will eat well meaning amateurs alive.