Trouble is, LF, that a docking station may itself cost 80-120 quid, and anyway, you'd effectively be paying for two screens - the one in the laptop and another for a docking station.
OP - Ebuyer.com has a number of Asus laptops at around 300 quid. I would aim for a laptop with a 15-17" screen, and then spend another 25ish quid at Asda/ Tesco/ similar for a wireless keyboard and mouse add-on.
Reason I suggest this combination - they keyboard and mouse would be similar in size to the ones you are used to on work PC, and you can place the laptop a bit further back on a desk, but fold it up and store it away easily as space is at a premium.
There's no need for a docking station, really, it's a bit of a moneymaker for the PC manufacturer (though I'm sure in some instances it serves well and looks a bit neater than having wires across the desk [though if you use wireless keyboard and mouse, there's no mess])
Why use separate keyboard and mouse - means you can stash the laptop away from the edge of a table so nobody can accidentally knock it off {and they should not get any crumbs or drinks spilt into it, either})... In the long run, also means that switching from laptop to laptop means no change in keyboard layout or how to use the mouse :)
All-in-One - they are quite neat, and having had a glance at the Amazon site, can see some in the 400-500 range. It might be better if you really want to do touch screen Windows 8, and it would look a bit neater than using a laptop.
I won an All-in-One in a website competition about 10 years ago and can see they have come a long way, but from a household security point of view, a laptop is much easier to put away, and if you needed to take it with you (eg visiting your parents, perhaps to show them photos or a video you had made) would be very easy compared with an all-in-one.
If you do go for an All-in-One, then given your (low) computing requirements, I'd go for the model that's priced at about 370, so you can buy an external hard drive and consider wireless keyboard/mouse. I know you don't plan to use machine to watch films but whether all-in-one or laptop they can play DVDs and many can be connected to a bigger screen (eg 32", 42" or more) via HDMI.
With wireless mouse and keyboard and computer connected to TV, you could find a video to watch and play it very easily on the big screen (or just watching YouTube or any number of other things :) )
As for MS Office - save your money and use LibreOffice or OpenOffice for free. Only bit missing is the MS Access (database) and frankly, if you need that, you would have to go for one of the top end Office bundles and pay 200-400 (sorry, lost track of current MS pricing!)