Hi Wayne,who'd have thought that UK gun laws would be more sensible than yours,go figure.
The use of moderators on PCP air rifles is almost universal over here,without them they give off quite a harsh bark,no problem at a range but if hunting it can be limiting.A lot of rifles come moderated off the shelf.
The most popular brands are Weihrauch,Air Arms and Huggett.
I use the Weihrauch on my .22 HW100 KT Weihrauch PCP
and the Air Arms Q Tec on my .177 S400 Air Arms PCP
I do not bother to moderate my only Spring rifle a Weihrauch HW35k in .22.
When moderated the rifles are very quiet and I would say cannot be heard at all
from 60 feet away.
We are limited to 12 ft lb's of muzzle energy (without them being on a firearms certificate) so our realistic max range for airgun quarry is 40 yards (for me 30yards) having a quiet rifle if a follow up shot is offered is essential.
Higher powered airguns are available over here but I have not come across any that offer the same power as say a .22 rimfire,although they do give very high accuracy and stealth.