Option 1: http://www.overclockers.co.uk/showproduct.php?prodid=LT-213-AS&groupid=959&catid=1828
Most bang for your buck, however it is an Asus laptop. I cannot speak from experience with regards to the ROG series, however their cheaper laptops tend to have problems with the plastic parts after prolonged heavy use (more than 4 years). They still work, they are just not very pretty. Still, this is the option I would recommend if you want to spend the full 1000.
Option 2: http://www.overclockers.co.uk/showproduct.php?prodid=LT-125-MS&groupid=959&catid=1828
Slightly worse video card, slightly better processor, less SSD space. On the other hand, a couple of friends have MSI gaming laptops and both are extremely happy with performance and build quality. The customer support also seems to be awesome.
Option 3: http://www.overclockers.co.uk/showproduct.php?prodid=LT-214-AS&groupid=959&catid=1828
This is a cheaper option - you save 200. Video card is the worst of the three but should still max out DotA 2 at over 60 average fps. If money is an issue and you really think you will only play DotA 2 (or less demanding games), I would go with this option.
Alternatively: Feel free to look around in shops close by and on the Internet. I am by no means very familiar with UK shops so these are likely not the best prices on these models you could find. Look for the following specs:
1) A 17 inch monitor is probably better (at least for me, personally). It makes the laptop heavier and sometimes more expensive, however the increase in size is really nice to strain your eyes less. Also, I would advise going to a physical store and trying different types of laptop monitors first - wLED is more common and generally less expensive, however the quality of the picture can appear different. Same goes for gloss vs matte monitors. Find the ones you like and look for them in combination with the other factors (or just make sure you can live with the difference).
2) 1080p is the resolution you want. You don't want less because it will look inferior, you don't really want more either because you would need more expensive parts to max out games.
3) Video cards (GPUs) - The minimum you need for comfortable gaming at max in DotA 2 is an Nvidia 860m - you might get some fps drop in really graphically intense battles but that should not be such a problem. Anything better is a bonus. If you want to be really sure you can max it out comfortably at 60+fps even in heavy battles, you need a 770m or a 870m. I am not at all familiar with AMD mobile GPUs so I cannot help you there.
4) CPU (processor): If you are following the guidelines for the GPU first, this part will take care of itself. Usually pre-made computers come with a much stronger CPU than what is needed considering the graphics card in it. A 4700hq is a good example. Oh yeah, you really want an Intel processor, considering this is a laptop and you want to play CPU intensive games (such as DotA 2).
5) RAM: It really doesn't matter all that much. You won't feel a real difference. Just make sure you have 8GBs (even though you can get away with just 4, I would strongly advise to go for 8). Don't waste your money on more/high performance - the difference is really not worth the money.
6) Storage: You can judge your needs better than I ever could. You can always supplement with external drives if push comes to shove. HOWEVER - SSD master race. Once I bought an SSD I have found it very annoying to work on computers without one. The feeling is so great it has to be felt to understand. Please note, this will not give you better graphics or more FPS. However, your game will start MUCH quicker and you will load MUCH faster (granted, you still have to wait for the other 9 people to load). It will also transform your entire experience with the computer and might get you addicted to it, making any other computer seem like a toaster in comparison.
You actually really need 128GB. Keep DotA 2 and any other games you play frequently on there, as well as programs and your operating system. The SSD is not for storage (such as music, movies, pictures) but rather for actively used programs and games! That said, the more SSD space you have, the more games and programs you can put on it. Use your discretion.
Final words: Sorry for the wall of text. I hope this is helpful. Have fun picking your new laptop and most of all, be happy with your purchase afterwards! Don't think on the lines "I should have bought this", just have fun with what you have. These considerations are for BEFORE making the purchase, not after :)
Disclaimer: I game on a desktop and have mostly been keeping up with news in that regard. Even though I used laptops for 5 years before that, my expertise is limited. Take everything with a grain of salt.