You'll find grayling pretty much anywhere on the Richmond stretches and the stretch above Brompton is as good as anywhere. Where exactly they are likely to be will depend greatly on the river level. You'll catch in places where the gravel/stones are normally dry if there's a few feet of extra water. Now that the dace and chublets have largely disappeared downstream, grayling will make up the bulk of any likely catches.
I'd suggest that you at least start by feeding very little or nothing. Even light baiting just tends to attract minnows in large numbers. Extended or heavier baiting will usually see the trout start to queue up. I catch the bulk of my grayling without feeding anything at all. Too much feed will either result in having to wade through the minnows or the trout bullying the grayling out. A perfectly good day can be had with half a pint of maggots. Don't be shy of using big hooks (10-14) with bunches of maggots to help avoid the minnows. Whilst I tend not to bother, small worms can make for a decent change bait. Despite many people advocating sweetcorn for grayling, I've never had a single grayling on it in about thirty five years of fishing in the area.
Much of the upper river (including all Richmond water) is largely pretty fast and shallow. At this time of year, I'd suggest looking at the more steady glides of around a couple of feet deep. If the water is low, then they might be lurking in the more broken water immediately after riffles. Chances are that it's not got cold enough yet for them to be lurking in the deep pools (usually on bends).
I'm a little odd in that I find proper stick floats, especially the lighter end of the spectrum, to be largely a waste of time on water of this nature. For those of a more conservative nature, I'd suggest Drennan Crystal Avons and Loafers. However, I'm more likely to be found using 3g+ Bolo floats or Avons with big sight tips. I find that there are times when it pays to trot a long way. There's not much point in that when you can't see your float. Catching a few fish will often see the fish dropping further and further downstream.
Langton is my usual stretch. I'm not suggesting that it's better than others, it's just closest to home. However, I'm still pretty familiar with Brompton and Easby. I tend to avoid the areas closer to Richmond simply because parking can be a pain. Have a good look at the
club website, it's now pretty informative, right down to pics, details and locations (use the What3Words app) of individual swims. The biggest issue we have at the moment is that our primary outlet for day tickets (the petrol station in town) burned to the ground a couple of months ago. However, if you contact the Membership Secretary directly, you can, by appointment, get day tickets direct from him.
Good luck, if you have any problems give me a shout. I'm not averse to guiding those in need of help. I wouldn't want to take anyone out wanting to catch chub or barbel, but I'm pretty confident of finding some grayling in all but the worst conditions.
@Sam Vimes might be your best shot for info on Richmond AA waters, theres day tickets available from the Catterick Bridge Hotel with a very good selection of water available from the road bridge and downstream
I was already typing! I doubt that the Bridge is still doing day tickets for their stretch, the hotel burned down several years back.