If you're looking to invest your money wisely cheaply made Chinese LEDs are not the way to go. Most aren't modular. Some will say they are just as good or made with the same parts, bullshit. (proven on the LED Constantinoplee links)
A good digital ballast or an A51 led or something similar from another reputable brand. I would not buy an LED unless it was modular had a good warranty and had next day shipping on parts. If your light goes out during flowering you're fucked you can't wait 2 months on China.
I wanna know how HPS's aren't safe but a knock off LED from China is.... If you're worried about safety buy a ballast locally that meets your countries electronic codes. Safety and Chinese knock off products should not be in the same sentence.
If you grow auto's indoors you are wasting your power. If you're in the UK Power is quiet expensive if I am not mistaken, growing auto flowers you need to run your lights 18/6 or 24/0 to get the best yield. While you can get better bud on a 12/12 cycle with photo's. So less power per day and better bud? You be the judge.
The bigger wattage you go with CFLs the less efficient they are. Run about 20-23 watt cfls. if you want to run CFLs. Don't run 1 powerful one they don't have shit for penetration and you'll still need side lighting, unless you scrog maybe, I still wouldn't do it with a high wattage cfl.
Did you read the reviews on the LED you linked? This isEXACTLY!!!!! why I say don't buy Chinese garbage. They don't give a fuck if you die or your house burns down.
You have a lot of reading to do before you waste £400.
If you have a 250watt hps it's perfect size for your space why would you not use it?
Best fertilizer recommendation anyone can give is build your own organic soil. It's not as hard as it sounds, and the results speak for themselves. The lack of effort to grow is insane, you add water. Maybe a worm casting molasses tea if you feel so inclined. Making the soils the hardest part, growing the plant becomes a cake walk generally speaking, still difficult strains out there that won't like what you made but when you find something that loves the mix you'll know it
For strains look at Female Seeds or Vision Seeds, both cheap but produced good results for me, if you fuck up it's no big loss. What I have grown from them has been really easy to grown.