Best UK Safe Treatment For Whitespot(Ich) Please?

I use King British original formula ws3. I only use this brand as I discovered it doesn't kill amano shrimps at half dose (I have loaches) when I couldn't catch them to put them in the QT for the duration. Most brands will do the job, provided you follow the instructions to the letter. There must be some medication in the tank when every last one reaches stage 3 of the lifecycle.
No matter how careful you are, a small amount of shop water will always get into the tank. I suppose the only way to try to avoid getting any shop water in the tank would be to acclimate the fish in a bucket, then replace the water in the bucket several times before putting the fish in the tank - and even then, there would still be the risk that there were the odd one or two stage 2 parasites (the cyst stage) in the bucket.

I aim for 29 to 30 degrees. It's only for a short while so they should be OK. The last time I had whitespot was from a new group of green neons tetras. These were very tiny too,  but they survived the treatment.