Short Version: my 11UK Renoirs are only slightly narrower than my 12E Strands/McCalisters (same last as Fifth Avenue). I could not go up to 11.5UK even if I wanted to, as there would be heel slip, and the laces would close together without the shoe being tight.
Being measured is very different from wearing & experiencing the shoes. But, the Fifth avenues are on the "5" last, which is considered a "narrow" last, so a 12E on the 5 last is only marginally wider than other "normal" lasts.
That said, I don't know what your fitting experience was like, or if you've had a chance to actually wear/try the shoes, so I'll just leave you with the following:
My first pair of AEs were size 12 McCallisters (same last as the Fifth Avenue). I was measured/fitted in store, and at the time felt great. But, after 2-3 hours of standing, I had to take the shoes off, as my feet were in TREMENDOUS pain. This pain continued for the next 5+ wears, until I finally had the shoes stretched. They are now wearable, but I would not call them especially comfortable. Since then I've added 2 strands in 12E and another McCallister in 12E, and they're much more comfortable.
That said, a good friend of mine was measured as a 9E, but he has to wear 3 tongue pads in his Fifth avenues, so something is very clearly off.
My Renoir IIIs (Soho last, same as the Winchesters) feel great in the ball of my foot, but do ever so slightly pinch my pinky toes. But, as the leather is much softer/more supple than AEs (which can be very stiff depending on what leather/color you get) the shoes are comfortably "snug" as opposed to putting me in pain.
Without knowing your foot size/fitting experience I'd say the following: If the 10.5Es were comfortable for you, get a 9.5UK. If the 10.5Es were still very tight for you, AND you have a high instep, only then would I recommend the 10UK/11US.