Sorry just realised I forgot to link the the guy still offering cibachrome:
http://www.owenboyd.com/
Never used him personally. I did get a 12x16" ciba done of a 645 slide by photech before the knocked it on the head, and it was good but they had to have a couple of attempts before they got the colour to my liking. I've seen a lot of joe cornish's cibachrome prints in his gallery/shop which were also done by photech, they wre all very good. Shame economic realities made them drop it.
I agree Peak do a good job, just when you get to shooting 4x5s the cost per sheet does make you wince, per square inch of film developed its much worse value compared to 120, hence why I stared doing my own E6. Peak E6 prices per unit area of film are getting on for three times as expensive for 4x5 as they are for 120, presumably cos of the overheads of the workload of manually handling and loading sheet film.
If I was still shooting soley 120/35mm I'd have stuck with Peak no bother, home E6 just isn't worth the hassle if you're only using 120. I still send them the odd roll or two every now and again, as I hate loading 120 reels.