UKC

Suppose I wanted to get on some of the classic Chamonix rock routes e.g. Cordier pillar, Charmox Grepon traverse. Is there a cragging venue in the valley suitable for having a go at mountaineering 6b with the odd peg/bolt? Or is all the easily accessible stuff fully bolted?

(While we're at it, what's the gear like on the Mummery crack? is there much scope for cheating if it turns out to be harder than planned?)

Aiguilles Rouge?

The stuff in the valley is mostly Gneiss & pretty much exclusively bolted - a nice retreat from the stress of the Alpine, but pretty far removed from any classic mountain granite rock routes. The Rouges is good, but again Gneiss, and you'll need a lift pass which would probably be better used getting on some less committing routes on the Plan (Nabot Leon, Papillons Arete) which will be much more specific prep for the routes you're aspiring to.

If you really are desperate to stay in the valley, Les Mottets or Les Dallas de l'Arveyron, (bolted single/multi pitch granite), or Les Chésereys (bolted multi-pitch gneiss) could be decent options

This? Ex-Libris (TD+ 6b)

Some different itineraries we've done on "off days" when we weren't doing a *big* objective:

(1) Run up the tracks to the Montenvers station, possibly stopping at the Buvette for an ice cream and sometimes visiting the glacier. Possible do as a loop from MDG campsite going through the town, or possible as a there-and-back. Can also turn into a scrambling trip by including the Via Corsa, and lots of other variations.

(2) Cragging at Gaillands 

(3) Spend a day up in the Aiguilles Rouges linking up multi-pitch routes (I've enjoyed La Someone, Crakoukass, and the Index)

(4) Go to the Aiguille D'argentiere and do some short pinnacle climbs

(5) Drive to a nearby multi-pitch venue like Cheserys or Vallorcine 

(6) Do a shorter route near your main objective like the Petit Charmoz traverse 

Some more suggestions here:

https://mtnath.com/alpinrunning-in-chamonix-valley/amp/

Most Cham mountain routes are "equipped" around the crux areas but may be well run out in other parts.  It's also worth bearing in mind that many of the classics are best described as being old fashioned in climbing style and requiring techniques that are very different from sports routes. I only mention this because you said 6b. Things like the Mummery crack may well feel very hard if you are not used to that style of climbing added to which you will be carrying a pack.

I was wondering whether the Mummery might be an exception and not be equipped around the crux? or does it have some fixed gear nowadays? I couldn't find info anywhere

Le Fantôme de l'Opéra (6a+)

Easily drivable from Chamonix, or take the train which is a little slow but very convenient.

This is pretty much the only trad at Barberine but there's plenty of sport, single and multipitch.

Edit to say: on a sunny summer day this would be quite unpleasant

Technically if you can climb cracks on grit Cham granite cracks will seem perfectly reasonable at a comparable  grade. The real hard climbs are the slabs,  regrettably out of fashion these days and a forgotten art. I checked an old diary and a bolted slab route at Envers that we did  I had graded sustained English 6B at E5/6. But I hear what you say about UK sport route grades. It would be a shock, especially when combined with the high mountain environment 

Absolutely and one of the reasons Joe Brown, Don Whillans and pals made such a huge impression on the area with new routes that astounded the French.  They of course cut their teeth on the grit outcrops.

Anything Perroux put up around the valley is pretty much guaranteed to be interesting, masses in the 5b+ range, all mountain crags etc...

Pseudo trad. Slightly easier than 6b but enjoyed it. No match for climb id:99463