Ignoring the US scene where they fly public aircraft, use sniper fire to knock out the engines of go fast boats [that have not fired back ... yet] and where they also think its great fun to whizz along and let off automatic fire at the 'target' [and every rabbit in the kingdom I guess] in the sure and certain knowledge that some poor sod will cop one - so not exactly sniper technology.
I am only aware of one instance where a sniper was carried in a UK police aircraft in 'anger.'
That was the Michael Ryan shootings in Thames Valley area in 1987 before the PAOC came in. A sniper was carried in the rerar seat of the AS350 hovering close to the school where Ryan was holed up. The only shooting was Ryan killing himself.