What is the safest phone?

All mobile phones sold in the UK should be safe, otherwise they would not be allowed to be released. The question is safe from what?

If you are refering to radiowaves and such, don't be too worried as these are not yet proven and nobody to my knowledge has been harmed by a phone because of this.

You should just use the phone properly and sparingly, maybe buying an older model if you are unsure of the modern features these days.