I’d tend to agree with the positive comments on Tyrwhitt shirts, plus their customer service (IME) is very good especially after they replaced a shirt after a few weeks when it broke(?). I would also unfortunately have to agree with their enthusiastic correspondence too though. Having said that, I’ve just got a £10 voucher from them.
Lewin shirts are decent but their delivery time and service is awful.
If you can afford them, Emmett shirts are fantastic but at £90 a pop they’re pricey. They do have very good sales in their shops, but unless you’re in London that’s not very helpful.
M&S are always good though, especially if you need to pick something up in a hurry.