Hey mate, i was in your position about couple of months ago. What i did was to spark *quite* a few spliff wiv a nice cuppa and start reading..
I agree with Arnold as BioBizz/Plagron soils is way overpriced, same as most Canna specific products.. this industry is shite mate.
Westland & JAB being your best and cost effective from local garden centres. I tried westland+ and liked it, but can't comment on JAB, but i'm sure Arnold recommendation is worth alot.
If you got the £ to splash and fancy sticking to the organic route i would second Arnold's suggestions and give Nature's Own soils a go .Don't want to spam so a quick google will point you to the right direction.
IMVHO the way to go for organics is to keep a healthy microherd. i would add the following to create a happy & effective microherd:
* Worm castings - Trace elements & microherd = happy soil = 10£
* Seaweed meal - Same as above = happier soil = 10£
* Rootgrow - To increase root mass = bigger yeilds = 10£
* Coco-Coir - Better soil structure = 5£
* Mollases / Treacle - To feed the microherd = 2£
* Bat guano - High P For heavy flowering
High N For vegging
* Rockdust - Trace elements
If you fancy keeping it simple for the first time, which will be very wise, i will stick with the first five and then move on to the rest. The list is just endless...
One final suggestion: Mix your soil at 1/3-1/2 strength as recommended on the products, as you can always add nutes to a weak soil oppose to soil which will burn ur plants cuz of too much nutes. Water, stir, and let it 'cook' (let the microherd do their job) for about 2-3 weeks in warmish temps.
This way most elements/nutes/goodies will be available for the plant straight away as the microherd already did their job.
Use OT1 feeding schedule with BioBizz Grow&Bloom (it is on here somewhere but i'm too blazed to check now.. sorry mate

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All can be sourced quite easily and they are not canna specific so won't burn a hole in your wallet. Google to source and search/reading in the Organic/Non-Organic Section will help alot
Good luck mate, you are going to love this hobby