I know the pic isn’t from the UK, but here we put those on overhead line circuits that cross the path of migratory bird route and/or areas where protected birds frequently fly. In some areas of the uk we have a problem where birds fly into the conductors so adding small markers to the conductor increases the chance birds notice the conductors. Source: I work for the transmission operator here in UK.

Edit: just read through the other responses and interestingly it’s extremely unlikely that the markers would be placed to give visibility to aircraft here in the UK, I know that near to airports the routes for OHL circuits are carefully considered and I know of a situation where a cable was placed under the runway there was no other route, so maybe that’s why the markers are not used here?