It depends on how much you think the laptop is going to be carried outside of the house. If you just want a laptop that mainly stays in the house then having a nice size screen is much better and great for watching TV/movies.
I meant 17" not 19" ....typo :P
Modern processors are more than enough for Office work, surfing and watching movies. You don't need hyper-threading unless you plan to run CAD software, Photoshop editing or any other CPU intensive applications.
4GB RAM is fine but 6GB or 8GB is even better. You can have more stuff open and Tab between without the disk having to page lots of memory.
Can you post a link to the laptop you are considering?
So is it worth another £30 for the i3 this laptop needs to last like 3 years? Or will the pentium be enough