I guess people missed the "UK" in the title lol.
Ignore amazon, you want to find yourself a local hydroponics shop. It may cost a little more but pay with cash, eliminate any paper trail. Just cover your ass.
What you need is a Secret Jardin DR900 or DS900, the DS if you are looking for cheap, as it is the older budget model. The 900 part of the model number is the size, so 900mm square floor space (external dimensions, so knock an inch off each side to get the internal space to allow for poles). The tent is going to be about £90. For that size, a 400w ballast will be suitable (£60-100). You will need a light fitting and reflector (£40-80) and 2 bulbs, one MH, one HPS (£20-35 each). Smallest fan/filter you can get is 4" which is more than enough for a tent that size (fan, filter and ducting £120) and you dont need any sort of intake for tent grows, it is all done with negative pressure (the fan sucks the air out, automatically sucking fresh air in on its own). All you need now is a HEAVY duty (dont go cheap on this) timer. They are about £15. That is your tent set up. With this set up you can swap the bulbs around and use the same tent for flowering as well. The tent will look huge and over sized and the light too much when vegging, but when you start flowering 4 plants and they nearly triple in size, you will see why you needed it all.
So tent set up is going to cost around £400-£450. But that isnt it yet. You need pots (cheap as chips) growing medium (use coco, its a good starter medium) and nutes. Use Canna nutes, as you can use the same for both flowering and vegging, so no messing about. Coco is about £15 a bag, 1 bag will do you for 4 plants easily. Nutes, i would recommend getting the 4 litre tubs (they come in pairs, A+B) for about £35 so it will last you, but its up to you. Then you need a PH meter and EC meter, which will be a minimum of £70 for the pair. These are EXTREMELY important. If anyone says otherwise, they are talking crap. You MUST check the nutrient solution you are giving to your plants. Now onto what you need for the plants...
Seeds. Order these from the net, as it can be difficult finding places to sell them in shops. I would suggest getting 5 seeds and expect 4 to make it. Sometimes you get one that is either a dud or a sickly plant. It is just luck. Get yourself a cheap heated propagator from the hydro shop, should be around £15-20, and some rockwool cubes for about 10p each. Use a cheap desk lamp from B&Q and get yourself one of the energy saving lightbulbs from tesco. Get the one with the highest lumen output you can find (if memory serves, i found 1100 easily) and stick it over the propagator. This will sort you out for the first 2 weeks, seedlings dont smell =)
I know it sounds expensive, and it is, for setting up 4 plants, but dont go for all this DIY stuff for your first grow. Get the gear listed above and you will probably grab an ounce per plant on your first ever grow in a 14 week period (2 weeks seed to seedling, 6-8 weeks seedling to full veg, 4-6 weeks flowering, depending how you want your high). So thinking how much an ounce costs here, it pretty much pays for itself in the first grow. After that, free weed. Happy days.
Oh and buy yourself a decent pair of pruning scissors, the expensive ones make all the difference. You wont use them in the first month but you will afterwards. Good luck.