I’ve used Tailorstore in the past and found them good. I got a white shirt in one of the better fabrics for about £70. It was very good quality – heavy fabric, good seams etc. You do need to have some time to wait for the shirts (4 weeks approx for delivery) and do some playing around with the measurements. You can also measure up a shirt you already have that fits and they produce a pattern from that.
I also had an M&S one a few years ago in their standard white fabric and it is pretty naff – fit is OK but the fabric is thin and the hem seams pucker. But this is to some extent reflected in the price which is think was around £25-£30.
Another of the online tailors does a free shirt in horrible fabric just to check fit – if you have the time to wait for it and adjust the measurements before placing your order, you should be able to get it spot on.
That said, if all you want is a standard shirt with longer sleeves, the TM Lewins and Charles Tyrwhitt types do that all day long. I’ve found Tyrwhitt shirts to be pretty decent, the non-iron ones almost are, and the standard shirts can be ironed pretty easily. I had some from Lewins and they went straight back – they were near enough impossible to iron.