Dive Machine in Tonbridge is a superb dive shop with very impressive filling kit, they do cylinder testing and maintenance in-house, and they also stock airgun filling equipment.
www.divemachine.com
There are other places in Newhaven and Brighton, but I've not used them. There is a list at http://www.channeldiving.com/dive_shops/dive_shops.html but its out of date, e.g. Hastings Scuba Centre closed down over a year ago.