I don't know why there's a £50 difference, because as far as I know they are the same thing. "Flat screen" is generally used for crt's to indicate that the screen face is flat. "Flat panel" is generally usd for crt to indicate that the whole monitor is a thin panel with a flat face.
I have studied the specifications for these monitors on various sites, and they are essentially identical, however on some sites the P9621T is offered as the budget version, but without any clarification as to what is omitted. The HP site is not at all helpful, only listing the P9621D. What is consistent is that they both have 1280 x 1024 pixel TFT displays. Unlike all the other sites, Inmac say that the P9621T can only acheive 60Hz, whereas the P9621D operates at 75Hz click here.
Note there are actually 2 models under the reference P9621T at Inmac (see Gongoozler's post above) What is wrong with the cheapest at £281.99. Can't see much between any of them to justify paying more. The P9621D is £101 dearer.
I think you may find the cheaper one is an earlier model that is no longer made, the dearer one is the model to replace it. The image does say for "Image for illustrative purposes only".
This happened recently with the AOC 17" TFT where there were two different prices on show for what appeared to be the same model but on closer inspection and a phone call to confirm it turned out that the cheaper one was the older model.
LCD and TFT are essentially the same when it comes to describing TFT displays. The type differences you need to look for are TFT +Film and APV or MPV as this refers to the type of panel used for the display. One gives a better, slightly sharper display bur with narrow viewing angles the other is not quite as sharp or suited to fast moving games but the viewing angle is much better.
Toms hardware guide is a good reference point for TFT comparisons. click here