Please do let us know how you make out...
Im in the same boat as you.
I am also a Charles Tyrwhitt customer. I'm not as slim as I was in my 20s. So it's classic fit shirts for me. They have been the best value in RTW shirts I have found so far (I am also a big fan of their ties). Shipping to the US is pretty quick, and, when they say no fuss returns, they mean no fuss returns. Some of the shirts arrived and the color looked a little off from the pictures on the website (probably because I need to calibrate my monitor). No problem. I just slap the return label on the box and schedule a pick up from USPS website. Done.
The CT twill and imperial oxfords are especially nice. Best of luck.
I was in Beverly Hills over the weekend. The SA from Turnbull & Asser told me that their RTW shirts drop to $100 a piece during their seasonal sales. They also have a store in NY. So, if you're cruising around, try on a RTW shirt. If you like the fit, you can get an excellent shirt during a sale event.