Hi Knocker...is it ok if I throw my two pennies worth.
I only started growing indoors last year after having grown nothing but photo's outdoors for years.
If your growing just for you and think 2/3 oz will last you then personally I would go for an 80x80x160 tent which will fit inside a standard wardrobe. I wouldn't grow in your loft for a couple of reasons, one of them especially seeing as though you live in the Smoke. You get a lot of helicopter traffic in London and the met have heat sensitive cameras on both their choppers, not a problem in the summer but in the winter a tent in the loft even with led's or cfl's will be warm enough to melt frost and give a heat source for the camera. Secondly going back to frost, in the winter if your loft is properly insulated then no or little heat will warm it from your house so when it's sub zero and frosty outside the inside of your loft won't be much warmer, plus as mentioned before it's a ball ache hauling water etc up a ladder.
as I said I would go for an 80x80x160 tent with a 300w dual spectrum cfl for your first couple of grows just to get the feel of indoor growing.
I use plant magic soil in 15l pots so that would suit your needs too. Location location location, I would try to put it in a wardrobe either in where the meters are or like me I have it in my bedroom with a padlock on the zipper to keep the kids out.
With the tent being 80x80 there will be enough room inside for extractor fan and filter so the only thing showing externally will be a little length of exhaust ducting, this will make the whole set up look more just like a storage unit, being just a big upright rectangle so to speak and make it a lot more hideable, I hang the bedspread over mine if we have guests round and it looks just like a piece of furniture with a cover on it
Strain wise, most auto's are now yielding 2oz upwards, I just pulled 4g over 3oz from one plant so I'm well happy with that as it lasts between harvests.
I hope this helps a bit, it all sounded so much simpler and shorter in my head....the joys of smoking your own bud, you can probably tell I'm a bit baked.
Anyway man if you want to know anything just give us a shout there's plenty of very knowledgeable friendly people on here who will come to your aid should you need it.
Happy growing and I look forward to your first journal