I think Thunderbird gets closest to the best way forward. All the stuff about the High Hedges legislation is correct, but that was intended against thoughtless neighbours who grow hideously high hedges once a neighbour is already in situ. The situation is different here - you've had the high hedge for many years and Planning Permission is about to be requested that changes the situation on the other side of the boundary. What you don't want is sometime later after the houses are up, a new resident getting the trees chopped under the High Hedges guidelines. They KNEW the trees were there when they bought the house!
Whilst you are unlikely to get (or even want) Tree Preservation Orders on conifers, you could seek to establish that any Planning Permission is granted conditional on the retention of the trees at their current (or something close to it) height. You should be a consultee in the Planning Permission phase, being so close to the Development, but it would be a good idea to approach the Planning Dept to see if such an objection at the PP stage might be received with a sympathetic ear.