Morning bro, here's two possibilities for your lighting setup from Vipar.
Since you are growing in a loft it's going to get cold in the autumn/winter/spring, you'll need to add a greenhouse heater for lights off and a couple of big 300w red cfl's would do the trick to keep it warm for lights on.
1st setup is for growing all 4 plants together from seed to harvest in one large tent.
2nd is for vegging 2 plants at one end of the tent and flowering 2 at the other end so you get a harvest roughly every 6 weeks. Will work the same in two smaller tents.
1. Due to your budget I've got two options, 3 of these
400w Reflector Panels at £148.49 each, spaced evenly along the tent as high as you can for the most coverage, they are a combination of 3w and 5w chips. Total true draw is 600w.
Or you could buy two of the 400w panels and add 3 x
300w 9U Red CFL's at £36.50 each. That would be CFL/Vipar/CFL/Vipar/CFL in a line which is what I would use having discovered how hard it is to keep the temps up with leds!
2. This is a much easier choice and would be my choice of using the space for harvesting either 2 plants every 6 weeks or one plant every 3 weeks.
For vegging I'd use one
400w Reflector at £148.49 and for flowering I'd use two
Vipar COB's at £314.98. Only £10 overbudget!
Hope these options help you bring it in on budget, you can always add cfl's or other panels once you have done a grow or have more cash free but you should be good to go for a respectable harvest with any of these options.
Oh and it's worth contacting Vipar to see if they'll do a deal with you on multiple purchases!