I’ve mashed at thicknesses of up to about 2.5 qts/lb. How much higher can I go with English malts before the mash is too dilute to get a good mash?
When I do 2.5 gallon biab I use the entire volume of water so maybe 4-4.5 gallons (16-18 qt with 5-6 # grain. I’ve done this with MO or English pale malt probably 5-6 times. Haven’t noticed any problems.
I have heard figures of 3-4 qt/lb as being the upper limit – beyond that, theoretically you might start to lose efficiency or extract tannins due to pH problems. So, to be safe, personally I wouldn’t recommend going above 3 qt/lb. Will you still make good beer if you go above that point? Well yeah, probably. But keeping it down under 3 qt/lb is good insurance if you’re not sure. If anyone has experience to the contrary, then please share! Let somebody else be the guinea pig! 
I BIAB and notice a dropoff in efficiency as I approach 4 qt/lb. So now if the full volume would be much thinner than 3-3.5ish qt/lb for the mash, I will stop there and top off with the remaining water in the kettle.
OTOH I used to BIAB at 1.5 qt/# and add the rest after mashing and got a sizeable bump in efficiency when I went from that to full volume. Given Eric’s experience above it seems likely that 3-3.5 which is where I’m at now, more or less coincidentally since my motivation was convenience, is a sweet spot… Its also much easier to stir a thinner mash. I suspect that despite my best efforts I was getting dough balls with my aggressive milling.
I frequently mash at 3qt/lb with no problems. There’s no upper limit - you can use your whole liquor volume to mash, as brew-in-the-bag brewers do. You get better efficiency if you sparge, particularly for high gravity beers. Thin mashes have higher pH so water treatment is advisable.
Thanks to all, I’ll try a mash ratio between 3 and 3.5. I’ll also add some US 2-row for insurance.