It looks like there isn't a firmware update for the UK version. When was that model released? It might just have the newest firmware already, as Seiki said they wouldn't do further updates for the SE42UM when I contacted them a while back. If it has a PC mode, you're pretty much set I think. See if you can follow my setup guide, and then if you still have issues it might be worth looking in to creating a modded firmware. Here's the link to the SE42UM's firmware: https://onedrive.live.com/redir?resid=9EC20EC148C073D7!851023&authkey=!ADgqAhYvYsKO3wo&ithint=file%2cbin
As for how you'd go about modding it, I have no idea. Careful you don't brick your TV since there's no stock firmware available for download!
Also, in regards to the lag, if you follow my guide successfully, you'll get about 50ms lag total. To put it in perspective, the QNIX QX2710 (a popular 1440p korean monitor) has 22-33ms lag, and not many people that own one even mention it. As I said in my review, it only matters if you need per-frame accuracy (CS:GO, LOL, etc), at which point you're looking at the wrong type of display altogether. I'm at the point where I can't even notice the lag with regular usage now. TBH, if you are getting "lag," it's probably a result of having a poor mouse and not because of the actual lag of the display. 4k+42"+crappy mouse=ugh. It'll just feel wrong moving your mouse around, dragging windows, precision clicking (eg small buttons), and so on. Irregardless of the model/brand you have, if you have a 4k screen of this size or larger, you'll want a gaming mouse just so you can traverse the display smoothly and quickly. One case where you won't need a great mouse is if you love super low mouse sensitivity and have a LARGE mousepad.
Nice lag chart:
25ms - pro gamers will notice if you A/B a 8ms with a 25ms monitor, but it probably wouldn't bug them too much overall. On it's own, a 25ms monitor probably would only bother INSANELY sensitive gamers, and they might not even be able to say with certainty it's the display's fault.
50ms - sensitive gamers would pick up a subtle difference between a 50ms and a 8ms monitor, and many standard computer users wouldn't be able to tell a difference at all. The standard users that can notice it probably wouldn't notice it until you deliberately point it out ("does it feel like it has any lag?").
75ms - many people will start to notice something is slightly off, the comment would be "things don't feel as snappy, not quite sure why"
100ms - Pretty much everyone will notice at this point, and then you'll start hearing "this TV has lag."
>125ms - display gets a reputation for having bad
lag. This is where the SE42UM sat for a while. The lag was so bad, it scared people away from ever trusting that "most of the lag is gone" means it's hard to notice the lag.
TL;DR: Lag won't be a problem
for you unless you're one of those people who needs a 8ms lag monitor at 120hz/144hz. Depending how sensitive you are, you might
be able to feel a slight
difference between the SE42UM and your normal monitors if you deliberately look for it
Also, just played Overwatch for the first time on this today and had a great time. Second game I ended up getting play of the game, yay! Yes, at 50ms this thing is suitable for heavy FPS gaming (but is your GPU? lol), which should say a lot. TBH, my biggest problem with FPS games on this monitor is that my GPU doesn't get over 60fps at 4k with graphics settings at Ultra. Typically I need to turn things down to High instead.