First, in most shoe brands I usually wear a US 12D. I own one CJ handgrade on the 337 last, but it was purchased from RL, so it's marked in a US size, 11.5D. It is long enough, but slightly tight (OK in the morning, somewhat uncomfortable after a 10-hour day). I also own a CJ benchgrade model, the Darlton wingtip also from RL, sized US 12D (it's a CJ Pembroke on the 325 last). So I go down 1/2 size from my normal US size for the CJ 337 last. When I order my next CJ handgrade, from PLAL, I'll order a UK 11E, to account for the difference between US and UK sizing.
I own one shoe on John Lobb's 8695 last in UK 11E and it's the best fitting shoe I own. For me, the CJ 337 last in US 11.5D is a tighter fit near the balls of my feet than Lobb's 8695 last in UK 11E, but the two are pretty comparable. I would say the lengths of the two are very similar and the width difference is a fractional size, with the 337 last being very slightly narrower. I hope this helps.